by William Voyek.
After investigating further I discovered that if I change the $CFG->directorypermissions from 00700 to 00707 the ClamAV scan will work.
However the $CFG->directorypermissions setting is identical for both my 1.9 install and my 2.3 install, 00700. Why does this work in 1.9 but not in 2.3? And what can I do to get it to work?