rafiqa

@tara, thanks for your suggestion. I have not done it, but I will do it because I want to know the culprit exactly.

In the mean time, I have figured out the way to fix the problem. I reinstalled WordPress and Buddypress from scratch using a new subdomain. Everything works fine. Buddypress 2.1.1 works fine in WordPress 4.1 in German as well as other languages. Sorry, my conclusion on the original post is wrong.

It seems to me that the internal server error occured because the update from WordPress 4.0.1 to 4.1 had been corrupt despite a successful update message. Because, even without the internal server error (when WordPress is in English), the TinyMCE Advanced plugin doesn’t work.