Hi filo,
I'm now pretty sure it is a Keymap problem.
My testuser "test" with password "z" can login with password "y"
"z" and "y" are swapped between us and de keyboard layout.
In your link above the last paragraph states at least wayfire uses us keyboard layout.
I tried
[input]
xkb_layout = de
in /etx/qtgreet/
config.ini / users.conf and wayfire.ini
But without success
thanks a lot!