Thank you very much for your reply.
I found the problem, it was a misconfiguration in the last line of the postfix master.cf in /etc/postfix/master.cf. The lines were not separated as they should be.
This caused a problem with spamassassin trying to deliver the mails to the named “users”.
I don’t know how this happened, but I did a fresh install. Something must have gone wrong with it.