Why the Internal Server Error 500???

I recently often get the Server error 500... I have a big thread and want add some pictures and text to the first posting in this thread. Once I click preview or submit i get this error.
Further I get this error if I want to embed animated gifs.

Does anyone know where it comes from.

There's a good sized thread on it right here.

https://www.forumophilia.com/topic100036.html