I would look in your logs for any related errors that might give a clue to the problem. Check both the mojoPortal log at Administration > System Log, and if you have direct access to the machine check the Event Viewer after producing the symptom.
No-one else has reported this problem, I do not think it is a bug. More likely a server configuration limiting the size of a request. You may need to limit it to a few files at a time.
Hope it helps,