How to use the app specific password?
How do you use the app specific password? Apple ID + app specific password does not work
Hello bern,
This document talks about using app-specific passwords --> Sign in to apps with your Apple ID using app-specific passwords - Apple Support
and a related article --> What is Sign in with Apple? - Apple Support
Of course there's the usual things to try such as making sure all your software is up to date and you have tried restarting your devices.
If you're not finding a solution there you may wish to contact the app developer if this is one particular app --> How to contact an app developer - Apple Support
If this is something happening with multiple apps then to me it sounds like a general interface issue and maybe contacting Apple would help --> Choose your country or region - Official Apple Support