Hello Jouko,
I am sorry for the late reply.
I have found the source to your issue regarding cannot post reply to the comment. It seems like this plugin:
System - Ads Starter Elite Plugin is causing this issue. This plugin somehow blocking/removing reply javascript on the comment. I have disable this plugin and now comment on your site can work properly. Here my screenshot of replies after I have disable this plugin:
http://screencast.com/t/ftcPX2vK . Can you consult with this plugin's developer and ask them to assist you on this issue?
If you have anymore question, please don't hesitate to ask us.
Hope this will help.
Thanks.