Having installed this plug-in, I received the following error when trying to publish a page:
http://wordpress.org/support/topic/352761?replies=5#post-1404172
Like stated, this could have been caused by the presence of 'WP Super Cache', in spite of it being disabled.
I really did notice the speed difference with this plug-in installed. I hope to try it again soon.
http://wordpress.org/extend/plugins/really-static/
Really static generates static files from your dynamic blog. Its imposible thats there no speed difference, beacause staticfiles are allways much faster than any file that is generated dynamicly.
I think you just activatet really static and didnt configuratet anything, or wrong. "FTP Logindaten falsch" means that your given FTP loginconfig is wrong.