The cause seems like having multiple connection to the same IMAP account.
I suspected from having multiple inbound threads, but it seems you configured only a single thread for imap. So this is not the cause.
Ensure that, you are using that imap account only from one WF mailer.. Each IMAP account should be dedicated to a single EBS WF Mailer.. Don't use the same imap account on different EBS environments.
Ensure you are not connecting to that imap account from different agents, and IPs.. WF Mailer should be the only one using it.
If you check the things that I mentioned above and if you think that you are okay, then check the OS side and IMAPD side.. You may be considering upgrading imapd and stuff like that.
See ->
https://bugzilla.redhat.com/show_bug.cgi?id=20206 , Bug 20206 - Lost Mailbox Lock in IMAP