No login via android app possible

Unfortunately, I haven't been able to log in via the android app the recent weeks since changing phone. The error is prompted as "der Server konnte nicht kontaktiert werden. Falscher Statuscode: 422".
I tried to log in from 3 different WiFi connections without success. Un- and reinstalling the app didn't solve the problem. Using another device didn't, too. Although the other device (my old phone) worked flawlessly until logging out.
Would you please get me instructions for troubleshooting?

Kind regards