several users reported problems when trying to submit new messages. In these cases error pages of MySQL have been shown up. Normally the message said something like
General Error SQL ERROR [ mysql4 ] Data too long for column 'post_text' at row 1 
Some investigation has shown, that this error only occurred, when either the message or the poster's signature contained some special characters. Further investigation has shown, that this error occurred after I have updated the server to the current MySQL version.
I now have (hopefully) fixed this problem. If you encounter any additional problems, please don't hesitate to contact me.