When you try to sign in any MS 365 app like OneDrive, Outlook, Word, etc. the signin works but it never actually signs you in.
You notice that the app is still not showing files or remains unlicensed.

This mostly has to do with the fact that older credentials are stored somewhere on the system. Even if you checked under Settings > Accounts in Windows 10 for example.
First you need to clear all the credentials

1. Start up a command prompt
2. enter the following command: rundll32.exe keymgr.dll, KRShowKeyMgr
3. In the popup that follows, delete all the credentials listed
4. Now start up registry editor
5. Go to the following path HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common and delete the entire Identity folder
6. Restart your computer and check your MS 365 apps. Log in if neccesary, it's possible that the apps automatically sign you in already