Same problem here...
When I try to sign in to my account, the "Bitte einen Moment warten" (please wait a moment) screen appears, then the program closes / crashes. This happens in all affinity programs (Photo, Designer, Publisher).
Did you buy the app from the Microsoft Store?
No, directly from the affinity website.
What version of Microsoft Store do you have running (found towards the bottom of the Settings page)?
-- (see above)
What's the version number of c:\windows\system32\RuntimeBroker.exe?
10.0.19041.746
Are you running any anti-virus software? If so, what is it?
Norton Life Lock