Ok, I found the problem! And it is a plugin! I don't know who is the developer of that thing, but all hikashop plugins contain the same DIRTY thing!
go to plugins/system/hikkashopuser/hikkashopuser.php
on line 19 you will see:
And basically - here is our empty line!!! All the plugins of this extension contain lines like this! In fact if you look at the source code of your page - you can see that the first line of your site is not <?xml ... but an empty line.
The solution? well either remove those lines
or disable the plugin.
Kind regards,
Daniel
P.S. The site was not working for me with the rt template. So I had to change it to a standard joomla one. and on a standard joomla template + disabled hikkauser plugin the comments were working. (just so you know)