Error occured in upload

I noticed in the seadrive log a line like this:

[09/07/18 15:34:40] sync-mgr.c(603): Repo 'folder-name-here' sync state transition from uploading to 'error': 'Error occured in upload.'.

But when I made a search on seafile logs with that timestamp (15:34:40), there is nothing error. How can I find out what the error is exactly, and its reason?

The Seadrive was 0.8.5 on Windows 7. It was running normally until last week.