Why can a user sign in on one device but not another?

Usually because the second device is still holding a cached credential from before the password changed, or because multi factor authentication is registered to a device the user no longer has. Check the account state before reinstalling anything.

What causes shared mailbox access problems?

Almost always a missing or recently changed permission rather than a fault in the mail client. Verify the permission actually exists and has finished replicating before you spend time on the user's profile.

How do you troubleshoot a licensing problem?

Check whether the account holds the license the feature requires and whether that license is still assigned. Features quietly stop working when a license lapses, and the symptom looks like an application fault to the user.

Guide

Common Microsoft 365 Help Desk Problems

By SysDesks Published August 30, 2026

Short answer

Most cloud productivity tickets are access problems rather than application faults: a lapsed license, a password change that has not finished syncing, a mailbox permission that was never granted, or multi factor authentication tied to a replaced phone. Confirm the state of the account before you start troubleshooting the application.

Microsoft 365 is a set of online tools many companies use for email, documents, and file storage. Because everything lives online instead of on one computer, a lot of problems come down to sign in, license, or connection issues rather than the program itself being broken.

Fix: the user cannot sign in

Confirm the username and password are typed correctly, including checking if caps lock is on.
Check if the account is locked or the password has expired in the admin system.
Ask if the person recently changed their password somewhere else, since old saved passwords can cause repeated failed sign ins.
Check if the account has the extra sign in step called multi factor authentication, and confirm the person still has access to that step, like their phone.

Fix: email is missing or not showing up

Ask the person to check their other email folders, since a rule may be moving messages automatically.
Check if the mailbox is full, since a full mailbox can stop new mail from arriving.
Confirm the account still has an active license assigned to it.
Check the online version of the mailbox through a web browser to see if the email is really missing, or if it is just a problem with the Outlook program itself.

Fix: Outlook keeps freezing or will not open

Close Outlook completely, then open it again.
Restart the computer, since this often clears out small errors.
Search for and open the Outlook repair tool through the Windows control panel or settings.
Check for updates to the Microsoft 365 programs.

Fix: OneDrive files will not sync

Check the OneDrive icon near the clock to see if it shows an error or a paused symbol.
Confirm the computer has an internet connection.
Sign out of OneDrive and sign back in.
Check if the account has enough storage space left, since a full account will stop new files from syncing.

Since Microsoft 365 involves the sign in system, the license, and the local program all at once, it helps to write down exactly which of those three you checked. That makes it much faster for the next person if the ticket needs to be passed along.

Common questions

Why can a user sign in on one device but not another?: Usually because the second device is still holding a cached credential from before the password changed, or because multi factor authentication is registered to a device the user no longer has. Check the account state before reinstalling anything.
What causes shared mailbox access problems?: Almost always a missing or recently changed permission rather than a fault in the mail client. Verify the permission actually exists and has finished replicating before you spend time on the user's profile.
How do you troubleshoot a licensing problem?: Check whether the account holds the license the feature requires and whether that license is still assigned. Features quietly stop working when a license lapses, and the symptom looks like an application fault to the user.