Seems like I have found the reason for this error. (And a fix!!!).
A time ago, I upgraded from version 6.x til 7.01. In that upgrade, I used the utility to convert the Access database as well. This convertion went fine, except, several people started reporting, they received mails from the forum about new messages in a thread they have NOT asked for.
I could also see in may mailbox, that my mailserver tried to send alot of mails from the forum to the users. And specially one particular user had an faulty email address in his profile. I (as administrator) edited his profile, removing his email address completely. From this day, the troubble started, (as mentioned in the above link).
Today I came to remember what I had changed, and I edited his profile again, and this time I inserted my administrator email adress insted. Now there is no more Internal Server Error 500 when entering a message in those two specific forums anymore.
I can reproduce the error, and fix it again (now I know how to).
-boRg-, this has to be a problem with the forum. What I did, anyone can do. Subscripe to Watch a specific forum (or thread) and then remove his email address from the profile. And it will result in the error again.
You might ask, why the hell remove the email address, when have subscriped to watch a thread/forum - Well, in this case I had to, cause the forum tried to send him a message all the time, and it failed, ending up filling my administrator mailbox insted.
I hope you can use this for something or a change in the next version. In case you want to see for yourself, just yell!
An suggestion - It would be good if the Administrator had an option to remove all Watch/Unwatch flags set for an specific user. This might have helped me in this case, rather than to edit the users profile/email address.
Edited by Andersen