It is possible Microsoft's identity key in the Registry is trying to connect to 2 different Microsoft/Office 365 accounts. For example: If you have added a SECOND account to Outlook and clicked all the way through without reading what was going on, this is most likely the case. To fix this:
(1) File>Account>under User Information, sign out all the existing account and exit all Office apps.
(2) Sign out of all Work Accounts under Windows "Settings->Accounts->Access Work or School" and under Control Panel>User Accounts>credential manager>Windows Credentials, remove all Office 365 credentials that look like: MicrosoftOfficeXXData:XXXXXXXXX and
(3) Right click windows icon>Run>type "regedit" then press ENTER to open the redistry editor. Go to HKEY_CURRENT_USER\Software\Microsoft\Office\x.0\Common\Identity (where x is the latest/used version installed), delete the whole Identity folder
(4) (This step may be redundant) Uninstall Microsoft Office
(5) Reboot computer
(6) Reinstall Microsoft Office
*******BEFORE MOVING TO THE NEXT STEP PLEASE MAKE SURE YOU READ THIS*******
Make sure you are signing in to Microsoft/Office 365 with only ONE and your MAIN licensed Microsoft 365 accounts while accepting your defaults. When signing in with the secondary account (in some cases, you may be signing in with this account FIRST!), make sure you click "No, sign in to this app only" in the following window! You may also uncheck "Allow my organization to manage my device" as this may force a sign-in by your organization on that account.
(7) Sign in to Office using your main Microsoft credentials by opening an app like Excel first
(8) When adding the secondary accounts to Outlook, please manually configure the settings to sign in to Exchange and NOT Microsoft/Office 365.
If any of the steps are missed above, you will likely need to restart the whole process again as Microsoft does not like two Microsoft/Office 365 accounts on the same computer.
**NOTE: If you are trying to use an Exchange account in Outlook that is not the account you are licensing the Office software with, it might benefit you to select "Let me set up my account manually" when connecting the Exchange account in Outlook and select "Exchange" like in the images below:
Microsoft Rep original instructions: