Comment 3 for bug 1784193

Andreas,
In /etc/systemd/system/clamav-daemon.service.d I have extend.conf.dpkg.old instead of extend.conf. That is where I got the two lines
ExecStartPre=-/bin/mkdir /run/clamav
ExecStartPre=/bin/chown clamav /run/clamav
from. Tjat would explain why i don't have the Drop-in line.
The packages I have are
ii clamav 0.100.1+dfsg-1ubuntu0.18.04.2 amd64 anti-virus utility for Unix - command-line interface
ii clamav-base 0.100.1+dfsg-1ubuntu0.18.04.2 all anti-virus utility for Unix - base package
ii clamav-daemon 0.100.1+dfsg-1ubuntu0.18.04.2 amd64 anti-virus utility for Unix - scanner daemon
ii clamav-freshclam 0.100.1+dfsg-1ubuntu0.18.04.2 amd64 anti-virus utility for Unix - virus database update utility
ii libclamav7:amd64 0.100.1+dfsg-1ubuntu0.18.04.2 amd64 anti-virus utility for Unix - library
Which appear to be the same as yours.
Looking in /var/log/apt/term file I see the line id:'clamav': no such user, but no other errors.

I would be glad to uninstall and re-install to see if /etc/systemd/system/clamav-daemon.service/extend.conf.dpkd.old has the right file name if you like.