Thanks, guys!
I have seen the response to the ticket, but I ran into the problem where my Steam Library won´t show up even after re-adding the Steam Platform. I then decided to make a Forum post, as SentaiBrad suggestet.
Jason Carr said
Adding the platform back actually won't help anything. The games have all had their platform removed. So the games will need to be deleted; they should still show up under All.
Oh my, now I feel kinda dumb! I didn´t even check under "All". I filtered the List, removed all the Steam-Games, re-run the Wizard and et voila, workes like a charm!
Thanks for the support, guys!