I found this report for a very similar problem with Fedora,
unfortuneately it's very unspecific about it's solution
("removing all software which had been installed from standalone rpm files" is
not really an option for my system;)
https://unix.stackexchange.com/questions/576828/fedora-31-kernels-5-5-10-and-5-5-11-fail-when-trying-to-decrypt-luks-root-filesy
BUT it'd limit the problems to somewhere betweeh 5.5.8 and 5.5.10-200.fc31
where can I find kernel RPMs with these versions for tumbleweed?