1. Log in to a VT (e.g. Ctrl+Alt+F4) and temporarily change your password to something simpler by running 'passwd'.
2. Change back to the graphics login screen: Alt+F1
3. Log in with your new simpler password.
4. Go to Settings > Language and Region > Input Sources > Manage Installed Languages
5. In the 'Language Support' dialog drag your preferred language to the top and then click:
Apply System-Wide
(Use the same language choices for startup and the login screen)
6. Now change your password again to something you prefer. And I hope the login screen will now accept it.
Possible workaround:
1. Log in to a VT (e.g. Ctrl+Alt+F4) and temporarily change your password to something simpler by running 'passwd'.
2. Change back to the graphics login screen: Alt+F1
3. Log in with your new simpler password.
4. Go to Settings > Language and Region > Input Sources > Manage Installed Languages
5. In the 'Language Support' dialog drag your preferred language to the top and then click:
Apply System-Wide
(Use the same language choices for startup and the login screen)
6. Now change your password again to something you prefer. And I hope the login screen will now accept it.