Default mail application unknown

Hey there, 

hope you are fine!

Im administrating ~ 180 users, without any issues when it comes to adobe.

One user is unable to send E-Mails from adobe directly. If he tries to press the envelope button, it lets him decide wether he wants to send a link or the whole file. Next step shows "an error occurred while sending mail".

In the settings i figured out, that in his case, it only says "Default E-Mail Application". All the other users see ""an error occurred while sending mail (microsoft outlook)" here. 

None of the other users really set it up. Adding the Account as new one isn't a possible solution, as you need IMAP and SMTP, which we dont want to use or setup within M365. There needs to be a way, adobe automatically recognizes the default mail app (outlook) just like it worked for everybody else.

What I tried so far:

- Reinstall Adobe
- Reinstall Microsoft 365 Apps
- Downloaded Microsoft 365 after authenticating with the user to portal.office.com and download it from there
- Reboot
- Install an older version of adobe reader
- Tried several registry-keys that were mentioned 
- Double checked outlook is registered as default app in Windows

Followed all the instructions of the first two google pages regarding this problem.  
Its excatl that problem, with the difference, that the solutions aren't working for us: 
Solved: Default email application (unknown) - Adobe Community - 10397697