Comment # 56 on bug 1106673 from Michael Hirmke

You are perfectly right.
In the rescue filesystem with the German keymap selected I get the same error.
So I changed the LUKS passphrase after booting the regular Tumbleweed
installation. I had to enter the passphrase assuming an english keyboard,
reentered it using the German layout - and everyting works as expected now.
Booting now allows to enter the passphrase using the German layout.
Thx!