aquafisch

Looks like a problem with compression (gzip)

But:
Fetching your page with wget gives me plain text.
It says
<!– Cached page generated by WP-Super-Cache on 2014-04-10 00:13:11 –>

Did you installed and enabled WP Supercache, too? That will not work!
Disable and remove this plugin. Clear all caches.

Have only one caching plugin installed and activated.

I agree with you, ptiper.

Found it.

First, the assests.php was not there when I looked at the directory, so i was confused. (I guess, my ftp client did not refresh the directory listing properly)

The problem was:
I installed wordpress in a subdiretory (https://codex.www.ads-software.com/Giving_WordPress_Its_Own_Directory) so the path in the assests.php was wrong. Had to add the name of the sub dir.

A suggestion:
do not use absolute directory paths in that file. Use something like DOCUMENT_ROOT or dirname( __FILE__ ) instead. (if wordpress or the whole server is moved on file system)

Next Problem:
After (re-)enabling WP Super Cache the tinyfier stopped working. It has no effect at the CSS and Javascript tags in my page’s HTML code.

Did you enable debugging? If yes, I’d say, this file is the debug log.

Open wp-cache-config.php and look for the filename. (The filename is removed from config file, when you disable debugging in the WP Super Cache config.)