And another user who's never contacted us before has now opened a ticket saying he received 10+ emails in his email client stating "The message is no longer on the server" and is now freaking out.

Are these users still seeing these emails in Webmail?

Checking with the users, they are also saying that the "Message no longer on server" emails are 1 hour in the future.

Josh, 1 user reported they didnt check and just removed the messages from their email client. However, currently no messages in their mailbox on webmail.

Good to know. Do you know if these are newer messages or older messages that are getting this message? Also if you could get me the cfg and grps from the inbox folder of an account that's having this issue, it would help a lot in troubleshooting what is going on. Thank you in advance.

No idea. Both have said it appears they are not missing any messages, but have no way to confirm this.