I can't see anything wrong with the messages. I've now found more messages that have been automatically removed, and can't find much in common with them.
Do they all have a body? I suppose they don't.
Some do, some don't. We are seeing this happen with very innocent-looking messages. I still can't pinpoint anything in common with them Is there any way to turn this 'auto-remove' off?
When I click on the "plain" link for any of these removed messages I get an OTRS error "Error: Can't read plain article! Maybe there is no plain email in filesystem! Read BackendMessage." Having a look in /opt/otrs/var/article/, that directory is completely empty. Should it be?
Depends on your settings. If you have: $Self->{TicketStorageModule} = 'Kernel::System::Ticket::ArticleStorageDB'; then all tickets will be stored in the database. to store (and find) them in the file system, you need this: $Self->{TicketStorageModule} = 'Kernel::System::Ticket::ArticleStorageFS';
What setting do you have? Supposingly DB, as otherwise you wouldn't have a single ticket in your system to view, because of the empty dir.
We have whatever is default, which I assume is DB storage, because we have quite a few tickets ;-)