hi werner!
excellent, glad to hear that disabling rokgzipper brought your site back! 
Note: This plugin is experimental and should be used at your own risk. You should not use the plugin during development stage and should only activate it on a finalised site.
Also, Gzip must be installed on your server and enabled in PHP in order to function.
www.rockettheme.com/extensions-joomla/rokgzipper
all the best, kat