It would be helpful if you filled out the rest of the support template. We don’t even have your config nor app list.
The one long entry you posted indicates you’re using files_antivirus, but it’s unclear whether that transaction is related to your situation. Is it during the time window of the deletions?
What else appears in the server logs during that time?
Your Desktop client sync clients should have some clues at the start of the syncing of those files too.
Any changes to files recently in that folder?
When was the client last updated? Just trying to narrow down what’s changed or may have triggered this for you.
Also, to confirm, you are checking the Web UI when you say the files are gone, correct? Not just in the Desktop client itself.
See: