Hey Jip,
I'm glad that you have figured this out, but please let me warn you. The joomla default cache is a mess.
It has a very basic usage - if you have a site that has only articles and you don't have any user interactions than you can use it. But in your case with comments - this is bullshit. When people login on your site, the cache is automatically turned off.
Also if you have cache, then google can't index the comments - as the page that is going to be served is the one with the ajax comments, and not the page that is made for indexing the comments:
developers.google.com/webmasters/ajax-crawling/
Regards,
Daniel