Hi Stefan,
There are a couple of items to try:
Try resetting Mathematica's settings. There are instructions at
http://support.wolfram.com/kb/12464
I would point out that if you try removing the User Base and Base directories, you first should back up your mathpass file located in the Licensing subdirectory of either your Base or User Base directories. This is your licensing information and you want to be able to put that back.
- Try downloading a fresh installer of Mathematica from the user portal and running it again.
If neither of the above fix the issue, please send this to support@wolfram.com. Through that venue, we could gather more information that may help us fix the issue but that you may not want to post in a general forum.
Sincerely,
Jason Grigsby
Wolfram Technology Engineer
