@Lucas Kanashiro (lucaskanashiro) 
Thanks for taking the time to read and answer it.

You're right, it is not a bug per se, more a serious shortcoming.
If we install the required packages, everything works fine.

However, if we modify one of the aforementioned folders to a symlink which points to another folder with the same contents, then the issue described in this thread pops up.

This is the first time I encounter this strange "symlink sensitivity" by any Ubuntu package and I believe this should never happen on Linux.

I understand that clamav is also implemented on Windows and this could explain this undesired behavior on Linux.

I have already posted a "feature request" on the <email address hidden> mailing list, but there is no response so far.

@Utkarsh Gupta (utkarsh)
Try this:
# sudo systemctl stop clamav-daemon
# sudo systemctl stop clamav-freshclam
# sudo mv -f /etc/clamav /etc/clamav.sav
# sudo ln -fsv /etc/clamav.sav /etc/clamav
'/etc/clamav' -> '/etc/clamav.sav'
# sudo systemctl restart clamav-daemon
# sudo systemctl restart clamav-freshclam
# sudo systemctl status clamav-freshclam
● clamav-freshclam.service - ClamAV virus database updater
     Loaded: loaded (/lib/systemd/system/clamav-freshclam.service; enabled; vendor preset: enabled)
     Active: failed (Result: exit-code) since Tue 2021-03-23 18:45:15 CET; 5s ago
       Docs: man:freshclam(1)
             man:freshclam.conf(5)
             https://www.clamav.net/documents
    Process: 2460871 ExecStart=/usr/bin/freshclam -d --foreground=true (code=exited, status=2)
   Main PID: 2460871 (code=exited, status=2)

Mar 23 18:45:15 host systemd[1]: Started ClamAV virus database updater.
Mar 23 18:45:15 host freshclam[2460871]: ERROR: Can't open/parse the config file /etc/clamav/freshclam.conf
Mar 23 18:45:15 host systemd[1]: clamav-freshclam.service: Main process exited, code=exited, status=2/INVALIDARGUMENT
Mar 23 18:45:15 host systemd[1]: clamav-freshclam.service: Failed with result 'exit-code'.

Then back to normal:
# sudo rm -f /etc/clamav
# sudo mv -f /etc/clamav.sav /etc/clamav
# sudo systemctl restart clamav-daemon
# sudo systemctl restart clamav-freshclam
# sudo systemctl status clamav-freshclam
● clamav-freshclam.service - ClamAV virus database updater
     Loaded: loaded (/lib/systemd/system/clamav-freshclam.service; enabled; vendor preset: enabled)
     Active: active (running) since Tue 2021-03-23 18:55:53 CET; 5s ago
       Docs: man:freshclam(1)
             man:freshclam.conf(5)
             https://www.clamav.net/documents
   Main PID: 2511510 (freshclam)
      Tasks: 1 (limit: 18975)
     Memory: 2.1M
     CGroup: /system.slice/clamav-freshclam.service
             └─2511510 /usr/bin/freshclam -d --foreground=true

Mar 23 18:55:53 host systemd[1]: Started ClamAV virus database updater.
Mar 23 18:55:53 host freshclam[2511510]: Tue Mar 23 18:55:53 2021 -> ClamAV update process started at Tue Mar 23 18:55:53 2021
Mar 23 18:55:53 host freshclam[2511510]: Tue Mar 23 18:55:53 2021 -> daily.cld database is up to date (version: 26118, sigs: 3965203, f-level: 63,>
Mar 23 18:55:53 host freshclam[2511510]: Tue Mar 23 18:55:53 2021 -> main.cld database is up to date (version: 59, sigs: 4564902, f-level: 60, bui>
Mar 23 18:55:53 host freshclam[2511510]: Tue Mar 23 18:55:53 2021 -> bytecode.cvd database is up to date (version: 333, sigs: 92, f-level: 63, bui>