@Nick-Carthew Thanks for the tip, Nick.
I already tried that (and just tried it again to be sure...) I even restarted my iPhone.
No effect.
When I tried to log in, I got stuck on the account-screen again.
I also tried to open the MyRoute-app via the option in the Route Planner (icon in the top right corner).
No success, however. I arrived at the "Account" screen and couldn't do anything.