Empty folders

I have a James 2 installation (Postgres) where a number of folders
just lost all their messages. The folders contain subfolders as well
as messages and for some reason after some time, the messages
in the top folder (subfolder of INBOX) just disappeared. A roundcube
mail client shows empty folders and Evolution mail client shows a
blue dot on all folders where the messages are missing. Is this a known
issue, and if so, is there a workaround to get the messages back?
Unfortunately backing up the Postgres database on a regular basis
is simply impractical because of the backup size.