Originally Posted by Lizard King
It happens if a member will try to send the message from quick reply and it happens usually on the first comment. Even the message character number is more then the vBulletin settings they receive this error.
When they switch to advanced reply then they never face a problem so I think the problem is same as you mentioned.
Edit : I believe it might be an issue with permission because i never faced this problem as administrator permissions.
Is your forum non-english ??
Have you enabled prevent-shouting ??