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Microsoft 365 is a suite of cloud-based productivity tools that relies on a combination of local software, network connectivity, and cloud-side configurations. When something goes wrong, the root cause could be anywhere in that chain — an expired license, a misconfigured tenant, a broken activation token, or a temporary Microsoft service outage. This guide walks you through the most common Microsoft 365 issues and how to resolve them systematically.

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Sign-In Failures

Sign-in problems are among the most frequent Microsoft 365 issues. They often stem from tenant mismatches, cached credentials, or Multi-Factor Authentication (MFA) configuration errors.

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Wrong Tenant

If a user signs in with a personal Microsoft account or an account that belongs to a different organisation’s tenant, they may reach the wrong portal or receive an access-denied error.
1

Verify the UPN

Confirm the user is entering the correct User Principal Name (UPN) — typically their work email address (e.g., [email protected]). Personal Microsoft accounts (@outlook.com, @hotmail.com) cannot be used to sign in to a work tenant unless explicitly invited as guests.
2

Clear cached credentials

Open Credential Manager (Windows) or Keychain Access (macOS) and remove any stored Microsoft 365 or Office entries, then attempt sign-in again.
3

Sign out of all accounts

In any Office application, go to File → Account → Sign Out, then sign back in using the correct work account.
4

Check tenant URL

Direct the user to https://login.microsoftonline.com and ensure they select the correct organisation when prompted.
If the user’s organisation uses a custom domain, the tenant ID can be looked up at https://login.microsoftonline.com/<yourdomain.com>/.well-known/openid-configuration — look for the issuer field.

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MFA Prompt Loop

An MFA prompt loop occurs when Microsoft 365 repeatedly asks for Multi-Factor Authentication verification without successfully completing sign-in.
1

Clear browser cookies and cache

In the browser being used for Microsoft 365, clear all cookies and site data for login.microsoftonline.com and microsoftonline.com. Then retry sign-in.
2

Try a private/incognito window

Open a private browsing window to rule out browser extension interference or session cookie conflicts.
3

Check MFA registration

Verify the user’s MFA methods are correctly registered at https://aka.ms/mfasetup. A missing or outdated phone number or authenticator app can cause loops.
4

Review Conditional Access policies

In the Azure Active Directory admin centre (https://aad.portal.azure.com), navigate to Security → Conditional Access and check whether any policies are conflicting or requiring device compliance that the user’s device fails.
5

Reset MFA for the user (admin action)

In the Microsoft 365 admin centre (https://admin.microsoft.com), go to Users → Active Users, select the affected user, and choose Manage multifactor authentication. Reset the user’s MFA state to Disabled, then re-enable it so they can re-register.
Resetting MFA will require the user to re-register their authentication methods on next sign-in. Notify the user before performing this action.

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Licensing Issues — App Greyed Out

When a Microsoft 365 application appears greyed out or unavailable in the portal, it usually means the required licence is not assigned or has not propagated.
1

Check licence assignment

Sign in to https://admin.microsoft.com, go to Users → Active Users, and select the affected user. Under Licences and apps, verify that the appropriate Microsoft 365 licence is assigned and that the specific app is enabled within that licence.
2

Enable the app within the licence

Expand the licence entry and confirm the toggle for the specific application (e.g., Microsoft Teams, Visio, Project) is turned on. Save changes.
3

Wait for propagation

Licence changes can take up to 24 hours to propagate fully. Ask the user to sign out and back in after 15–30 minutes as a first check.
4

Check group-based licensing

If licences are assigned via Azure AD groups, ensure the user is a member of the correct group in Azure AD → Groups. Group membership changes can also take time to sync.
5

Verify service plan conflicts

Some licences include duplicate service plans (e.g., Exchange Online Plan 1 included in both a standalone licence and an E3 licence). Conflicting service plans can cause apps to appear disabled. Remove duplicate licences to resolve the conflict.
Use the Microsoft 365 Admin Diagnostics tool built into the admin centre. Navigate to Support → Run diagnostics and search for “Licence” to run automated checks against the affected user’s account.

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Activation Errors

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Error 0x8004FC12

Error 0x8004FC12 is an Office activation error that typically indicates the product cannot contact the Microsoft activation server or the licence is in an invalid state.
1

Run the Office activation script

Open an elevated Command Prompt (Run as Administrator) and navigate to the Office installation directory, then run the activation script:
Replace Office16 with the appropriate folder for your Office version (Office15 for 2013, Office16 for 2016/2019/2021/365).
2

Check the licence status

While in the same directory, display the current licence information to confirm whether the product key and licence are valid:
3

Verify internet connectivity

Ensure the machine can reach Microsoft’s activation servers. Test by browsing to https://activation.sls.microsoft.com from the affected device.
4

Run the Office Repair tool

Go to Control Panel → Programs → Programs and Features, right-click on Microsoft 365 Apps, and select Change → Quick Repair. If Quick Repair fails, choose Online Repair.
5

Sign in to activate

Open any Office application, go to File → Account, and ensure the user is signed in with a licensed account. Click Activate Product if prompted.
6

Remove and re-add the product key (volume licence environments)

If the device uses a volume licence (KMS or MAK), uninstall the current key and install the correct one:
The ospp.vbs script is located in C:\Program Files\Microsoft Office\Office16\ for 64-bit installations, or C:\Program Files (x86)\Microsoft Office\Office16\ for 32-bit installations on a 64-bit OS.

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OneDrive Not Syncing

OneDrive sync issues within the Microsoft 365 ecosystem are common. For quick resolution, ensure OneDrive is running (look for the cloud icon in the system tray) and check for any red or yellow status icons indicating errors.
For a full, step-by-step guide to resolving OneDrive sync problems — including common error codes (0x8007016A, 0x80070194), file path limits, and account relinking — see the dedicated OneDrive Sync Troubleshooting guide.

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Outlook Connectivity Issues

Outlook relies on several protocols and endpoints. Connectivity problems can manifest as “Disconnected”, “Trying to connect…”, or “Need Password” status messages.
1

Check the connection status bar

In Outlook, look at the bottom-right status bar. Hold Ctrl and click the Outlook icon in the system tray to open Connection Status, which shows each active connection and its state.
2

Test AutoDiscover

Hold Ctrl and right-click the Outlook icon in the system tray, then select Test E-mail AutoConfiguration. Enter the user’s email address and click Test to verify that AutoDiscover can locate the Exchange server settings.
3

Check proxy and firewall settings

Ensure that the required Microsoft 365 URLs and IP ranges are not blocked by a firewall or proxy. Reference the official list at: https://docs.microsoft.com/en-us/microsoft-365/enterprise/urls-and-ip-address-ranges
4

Create a new Outlook profile

Go to Control Panel → Mail → Show Profiles → Add, create a new profile, and configure it with the user’s Microsoft 365 account. Set the new profile as default and relaunch Outlook.
5

Flush DNS and renew IP

Network-level issues can prevent Outlook from reaching Exchange Online. Run the following from an elevated Command Prompt:
6

Run the Microsoft Support and Recovery Assistant

Download and run SaRA (Support and Recovery Assistant) from https://aka.ms/SaRA. It automatically diagnoses and fixes common Outlook and Microsoft 365 connectivity problems.

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Checking Microsoft 365 Service Health

Before spending time troubleshooting, always verify whether the issue is on Microsoft’s end.
1

Open the Service Health dashboard

Sign in to the Microsoft 365 admin centre at https://admin.microsoft.com with a Global Administrator or Service Administrator account.
2

Navigate to Service Health

In the left navigation pane, select Health → Service Health. This page shows the real-time status of all Microsoft 365 services.
3

Check for active incidents

Look for any services marked with a yellow (advisory) or red (incident) status. Click on an affected service to see the incident details, estimated resolution time, and any workarounds Microsoft recommends.
4

Subscribe to email notifications

Under Service Health → Preferences, enable email notifications for service incidents so your team receives alerts automatically.
You can also check the Microsoft 365 status page without admin access at https://status.office365.com. For historical incident data, the Message Centre in the admin centre records past communications.

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Escalation

Microsoft Support

Raise a support ticket through admin.microsoft.com → Support → New service request. Provide the affected user’s UPN, the error message or code, and the approximate time the issue began.

Microsoft 365 Admin Centre Diagnostics

Use built-in diagnostics at admin.microsoft.com → Support → Run diagnostics to auto-detect and resolve common account and licence issues before escalating.

Microsoft Tech Community

Search for similar issues and post questions at https://techcommunity.microsoft.com for peer and Microsoft engineer responses.

Service Requests

When escalating, include: tenant ID (found in Azure AD → Overview), affected user UPN, error code or screenshot, and the correlation ID from any sign-in error page.
Never share Global Administrator credentials with Microsoft Support. Microsoft engineers can access your tenant using their own elevated access tools when a support ticket is open.