Thanks Giovanni for the help! It is PostgreSQL + RabbitMQ. The aiida version i am using is 2.4.0 with a “special patch” from Sebastiaan from a while ago (this is the discussion: Excepted workchains, due to strange error from kiwipy/plumpy?).
I don’t know whether that fix has now been implemented in the latest main branch of aiida-core.
The error though seems kind of random, depending also on the rest of the daemon workload, and even running using caching to skip the steps the behaviour was quite different each time. So, I guess it’s probably related to the version of aiida-core i am using, and by the fact i am running quite heavily on my workstation (?).