https://compojoom.com/support/documentation/ccomment/ch06#id1332049

Hi Daniel, thanks for the link. I checked this out, but I do use only conservative caching. Not progressive as mentioned in the 'known issue'. So also with conservative caching on the comments do not work.

Hi Daniel,
The problem was caused by Admin Tools Pro. I had to allow the comments component to access the cache folder. Now it works fine.
In the .htaccessmaker of Direct Admin:
Add '/cache/com_comment/' to the field 'Allow direct access, except .php files, to these directories'.

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: https://developers.google.com/webmasters/ajax-crawling/

Regards,
Daniel