mailbox - Failed to lock mailbox

Hi.

I had to upgrade ZeXtras on one of my server (running 8.0.7) because the old version was not able to reach the licence server.
Since then, I'm seeing a lot of these errors "mailbox - Failed to lock mailbox" on some mailboxes.

I upgraded ZCS to 8.0.9 (and re-installed ZeXtras), same problem.
I added RAM to the server (from 4 to 8 GB, for about 320 users doing only a few mails pushes), same issue.

It seems the issue is not unknown... Any clue?