The hosting company could help me. I turned out that I needed to temporarily deselect a safeguard against cookies in my browser (Safari 8.0) to access the content through their website. That feature interfered with the login through FTP.
After that, I could locate the plugin-folder, delete it (made a backup first, of course) and that did the trick. The problem has been solved.
Additionally, the plugin changed the code for all my embedded youtube videos so that they scale properly on mobile devices now. Therefore I can use that code to scale all my future embedded videos with the same code without the plugin ??
Thanks and goodbye.
