It won't help in backtracking an existing issue, but you can turn on extra logging for specific users, which apparently will last until the next server restart. We ran the following:
Code:
zmprov aal <username@domain> zimbra.mailop debug
Zimbra Support also suggested we add
Code:
zmprov aal <username@domain> zimbra.soap debug
From there, it's a matter of waiting until the user in question reports an issue that occurs and hope you can see some information to use (or provide to Zimbra Support).