Just wanted to note this. I kind of messed up analyzing this. I returned from a multi day freeze of my VM, so there was a timejump, and got a general error signing / encrypting this was repeatable. Strange thing though in the diagnostics there was no error. I then tried on the command line with --verbose but messed up my test and created a different error. After that I saved the pubring.db and wanted to restart keyboxd with debugging enabled.

Why I blame keyboxd is because as this is the newest component I suspected it and
I only did gpgconf --kill keyboxd
and kept Kleopatra and gpg-agent running. And then it worked without a problem.

Prio low at least until this report has more qualified information.