Today it suddenly decided that a DLL in a piece of legitimate software was a virus (almost certainly a false positive),
what virusname did avast give the file?
What program did it belong to?
Sometimes file cant be quarantied, it may delete if not able to quarantine.
If you think this is a false positive you may report it here
https://support.avast.comInfo: next time, use
viruses and worms forum section for reporting virus/false positive problems