Unfortunately the keyboard layout not being set correctly on the login screen is a known issue.
You can probably type this on the login screen as follows:
- Hold
Ctrl
+Alt
and pressu
, then release Ctrl and Alt - Type
f
then6
- Press space to accept the character
The other way you can work around this is to try using an incorrect password a couple of times – this will trigger a password-reset flow.
Once you have managed to get back into your account, you can follow the steps on Can't switch to AZERTY layout on login screen - #2 by wjt to work around the problem. Assuming you use a standard German keyboard the incantation you want is:
( echo XKBLAYOUT=de ; echo XKBVARIANT= ) | sudo tee /etc/default/keyboard
then reboot.