“There was a general problem while updating the user’s signatures and settings during the operation”
A very generic error message on a client machine running EXSYNC.EXE which other than above appeared to be ok. The server also was giving no errors, and policy tests worked.
The issue, was simple, but not simple to find. Because the client machines used to be connected to an internal Exchange server that had been retired, and the mail moved to Office 365, the OWA settings that existed would not work. By removing the updates on the disclaimer rule to update OWA and Outlook, to just Outlook, this issue went away.