We had a client with a forum where no more private messages could be sent. Any attempt would result in a 500 Server Error.
This turned out to be caused by the fact that
min_post_length was set to 0, resulting in a division by zero in
Offending code on line 18
scale_entropy = SiteSetting.min_private_message_post_length.to_f / SiteSetting.min_post_length.to_f