I have seen in the security logs of this server, that some of you got problems with sending PMs and get a 500 page.
On this server is
Apache Mod Security running. This blocks almost all SQL injections, XSS attacks and many more. If you use code in your PMs or SQL statements or something that looks like code it could be this messages are blocked.
I don`t wanna change the rules for ModSecurity if it is not realy neccesary, cause if i see the up to 100 attacks a day and what these script kiddies all try to do i get scared

Ah, not realy, but it`s better to keep the rules.
So take care what you are writing.