I've found the problem:
I used the following code for one of the pictures:
<center><img src="https://dagwacht.duclavier.com/PoeF20000/De%20proeflopers.png" /> <img /></center>
The latter <img /> was wrong.
It was there from the start.
Thank you all for your support. Learned a lot today.
Going through the procedure of sending a bug report I found this.
The error 500 is mentioned specifically.
The best way to report a bug is in the forums.
First see if the issue has already been reported.
Add new information to an existing thread.
Start a new thread if the bug has not yet been reported. Be sure to use a specific topic title and description. For example: "500 error when editing cache page"