CI 4.18 -> 4.20 causes RouterException

I think I've found the problem...
It's an url_to() issue.
I have this line to redirect to the dashboard when a user is already logged in:

PHP Code:

return redirect()->to(url_to('\App\Controllers\backend\Dashboard::index')); 


This line throws the error.
But when I change it to:

PHP Code:

return redirect()->to(url_to('backend\Dashboard::index')); 


It works, but I have many uses of url_to(), it would be quite some work to modify all these occurences, and with a risk of forgetting one.

Does anyone know, why this is a problem all of a sudden?
And is there a way of making it work like it used to work?