User (license) accounts are often tied to (or 'associated with') soft Tokens stored on the computer, or you may also look at is as a traditional Cookie (token) with log-in Session information "carried" inside the Cookie. I have recently done some troubleshooting of similar issues with logging on to a 'Virtual Machine' instance, and found between 5 and 17 Cookies were being created and 'associated' with the log-in Sessions.
If the User knows this is their first time logging in to the web-based or Cloud-based resource and the attempts were (all) made within the last HOUR, then going into the browser's 'Settings' and deleting all the Cookies from the past HOUR, closing and re-launching the browser and attempting access again MAY do the trick! It sounds like a simple answer, and some times the simple answers ARE (do prove to be) the best ones.
Missing a Mailbox or a License ('Error: 500') is a different issue and the previous advice about contact the business' I.T. Administrator or Help Desk hotline is VERY good advice; something on the Backend needs to be checked which the End Users are unable to see themselves. A software license may have a (1.) limited number of 'seats' or Users, (2.) may have run its course on pre-payment such as a 1 year or 3 year license, or (3.) may have been 'Suspended' for some other reason, or {guessing here} (4.) the Users may have a geographically-locked license which is only good in 1 country and now the User has traveled abroad and into a non-covered area. I mean, it could be an intentional 'restriction' of sorts.
Conflicting licenses is another issue entirely and 'Yes!' I think it is possible for a web-browser to be storing and submitting the 'incorrect' license for a user. So what is the quick fix here? ... If permitted, Download another browser and attempt access to the online/cloud-based resource using it! Microsoft Edge (tm) stores its Cookies and Tokens in one area, and another web-browser will DEFINITELY store its Cookies and Tokens in a separate area, so if browser 'A' isn't working, try getting and using browser 'B'.