We’ve received confirmed user reports lately that new mailboxes do not work with the current version of RoundCube when deleting mail. RoundCube replies with an error, that it cannot move the message. It doesn’t say “to the Trash folder” but that is in fact the problem. When a Trash folder is manually created, everything works fine.
This is either an issue of RoundCube not automatically creating the Trash folder or that InterWorx is no longer creating the Trash folder. I’m really not sure which package is at fault.