Copying by IMAP Failing with UID Copy Failed

Copying by IMAP Failing with UID Copy Failed

I'm using IMAP with this account. With two different mail clients, Postbox 5.0.19 for Mac and Mac Mail 10.3 I'm getting the error "UID COPY failed". This happens either when deleting (copying to Trash) or copying to any folder. 

Seems to be working fine from the web interface.