Hey there Lennart, sorry to hear you're having these POP issues! Since the user has several other working accounts in this client (I'm assuming all on the same server?) this would lead me to believe there may be an underlying profile issue involving that particular account in the user's email client. If they sync their account to a different email client on another machine (make sure to turn on leave on server so it doesn't remove their email!) does it throw the same errors?

The reason I ask is because POP is a one-way sync protocol, so once the email is downloaded there is no further server interaction at that point. Now, if the error is happening while they are POP'ing down their email there may be a message in the Inbox currently that is corrupt or otherwise problematic. Please don't hesitate to submit a ticket with us if you need a hand! I hope this helps :)

Ok. Thanks @Kyle
As a matter of fact they have managed to download again after manually deleting all in Inbox.

What curruption could it be?
A mail with a virus?