System volume root's usage is 90%.

# du -csh /mnt/*
1.1G /mnt/var
1.5G total

This suggests the problem is in /var. So you could do e.g.

# du -csh /mnt/var/*

and so forth until you find some big files e.g. some big log files.

If in doubt, ask whether something is something you should empty or remove.

If the anti-virus service is still running you should stop and disable that service. Maybe I was wrong and there could be some huge anti-virus related files still there.