SO, my qnap stoped updating clamav after firmware update.

check: ldd /usr/local/bin/freshclam

if it seems to be missing som libs

fix:

ln -s /share/CACHEDEV1_DATA/.antivirus/usr/lib/libclamav.so.9.0.2 /usr/lib/libclamav.so.9.0.2
ln -s /share/CACHEDEV1_DATA/.antivirus/usr/lib/libclammspack.so.0.1.0 /usr/lib/libclammspack.so.0.1.0
ln -s /share/CACHEDEV1_DATA/.antivirus/usr/lib/libclamunrar.so.9.0.2 /usr/lib/libclamunrar.so.9.0.2
ln -s /share/CACHEDEV1_DATA/.antivirus/usr/lib/libclamunrar_iface.so.9.0.2 /usr/lib/libclamunrar_iface.so.9.0.2

ldconfig

freshclam -u [nasuser]

link for info: https://forum.qnap.com/viewtopic.php?t=95649