My problem was, that the keyboard starts after a shorter time pressing down a key with repeating keystrokes than in the normal desktop environment.
This makes it more difficult to correctly enter the passphrase, requiring extra short keystrokes.
After all, in my opinion the repeat keys function makes no sense in an environment without visual feedback, so I wanted to get rid of it.
Since at startup, not the normal Linux is running, but initramfs, the normal settings don't apply here.
The problem is discussed at several places, but no solution worked for me, like these: