And you have added all the users, that need access to the users group in /etc/group?IE your /etc/group file contains a line like: users:x:100:user1,user2,user3,user4,useretc
Yes.
If so, than it is spamassassin that does not switch the user context correctly.
It looks a bit like it. I've just installed a newer version of SA from backports.org. Trouble is that other tests seem to pick up spam before FOCR is invoked. I've tried increasing focr_autodisable_score - I'll have a look in the morning.