Might have been a folder naming issue, but reports currently working after upgrade to 5.0.5 (before more troubleshooting was done).
Anyone else hitting this error start by:
su - zimbra
zmprov aal
user@domain.com zimbra.datasource debug
You should see a "Processing IMAP folder" message in /opt/zimbra/log/mailbox.log before the failure occurs that will tell you the name of the folder.
Try renaming the folder and see if that solves the problem. To rename the folder you can run:
zmmailbox -z -m
user@domain.com rf {folder-path} {new-folder-path}
For example,
zmmailbox -z -m
user@domain.com rf "/Test" "/Test1"
To turn off debug logging, you can run:
zmprov ral
user@domain.com zimbra.datasource
Note: account logger setting will also go away if you restart the server.
If renaming the folder fixes the issue, please let us know the old folder name and we'll file a bug. If renaming the folder doesn't help, attach some mailbox.log snips with zimbra.datasource debugging enabled.