[exim] Strange error from ClamAV

We run identical installations of exim 4.53 on three SMTP servers.
Recently, one has started rejecting all email, with the log showing
messages like



malware acl condition: clamd: ClamAV returned
/var/spool/exim_incoming/scan/1EMk73-0001ij-Ac: Access denied. ERROR



This looked like a permissions problem, but the protections on the
/var/spool hierarchy are identical on the failing machine and the two
working ones. As a test, I chmod'ed 777 all the way up the hierarchy,
but the problem persisted.



I am baffled ... can anyone suggest what might be wrong?



Thanks



Alan Stanier

Essex University