Comment # 5 on bug 1201439 from Dirk Weber

Just a suggestion from a wild guess, maybe it is helpful...

in Leap 15.4 the keyboard layout for entering the passphrase is originally
localized (e.g. "de").

Does your passphrase contain characters which are located at different keys for
keyboard layout "de" and keyboard layout "en"?

Some weeks ago (?) it happened in Tumbleweed that the localized keyboard was
not setup correctly for grub to enter the passphrase. Maybe something similar
happened now resulting in an invalid passphrase when you think you entered it
correctly.