outlook wont let me add an IMAP Mailbox for a specific User

Hey i know this problem sounds very trivial but it cost me some hours already and I wont ever post any question into a microsoft forum again because the feedback and responses you get are just not that helpful.

I just want to try to add an IMAP mailbox for a specific User because that user needs it. But outlook responds with a popup saying:

"Something went wrong, we couldn't create your outlook data file. Make sure the disk isnt full or write protected and is not in use."

None of the adressed issues are the case. I was able to add her personal IMAP mailbox previously. I however am not able to add any other IMAP mailbox. However exchange works. The strange thing is, when loggin in with my profile, I can add said IMAP Mailboxes to my outlook profile.

Anybody have a clue on how to resolve this? I already created another outlook profile for her and that didnt help. I restarted the Terminal Server and that didnt help. The outlook version is 2019 and the server has all updates.