I saw in Boris logfile that the log is partly in german as well as my log on a Debian Bullseye using rspamd 3.2.1 on a RockPro64.
I opened an issue for yunhost which is the system integration I use on that system on the basis of Debian here: https://github.com/YunoHost/issues/issues/2269 "Switched the locale back to de_DE.UTF-8, restarted rspamd and reproduced the bahviour. To get my ynh working with rspamd I switched back to C.UTF-8, restarted rspamd and got the same errors containing german again. WTF. After I restarted the daemon a few times and tried after each time to learn more ham/spam from existing folders it suddenly started working without crashes again. I guess someone needs to look at the problem on their host to compare results."