Low-bandwidth mode makes Opera only fetch part of a messages by default. The rest doesn't get fetched until you right-click the message and choose "fetch complete message".
So, it could still be part of one or more messages that's causing the problem. You might run into an error when fetching the complete body for one of the messages that came in. Not sure what the deal is, but logging could still help. Is this with IMAP or POP though?