Probably late, but i just fixed this issue on my server (debian), the clamav-daemon.service file need to have JoinsNamespaceOf=apache2.service on the [Unit] and PrivateTmp=true on the [Service] , like this:
[Unit]
...
JoinsNamespaceOf=apache2.service
[Service]
...
PrivateTmp=true
The apache2 file just need to have the PrivateTmp=true. Do a systemd daemon-reload after editing the systemd files and restart the clamav-daemon.