does your host have AV on the server, that might corrupt the .grp file?
the message only occurs when one is using pop to download messages, but the message isn't there/not readable/corrupt, or locked by the AV?
So ask your host about the setup.
I have 1400 users, haven't had this problem. I have no AV on the server, turned off windows defender (Windows 2016 server) I only use the ClamAV delivered with Smartermail.