Joomla 1.7
K2 2.5.7

If I set the comments option to "Enabled for registered users only"

A top level category page works fine, ie:
Navigation is: News > Category > Item
xtcgaming.co.za/index.php/news/xtc/item/188-xtc-take-on-the-do-gaming-championships-at-rage-2012

But a sub-category page breaks:
Navigation is: News > Category > Sub-Category > Item
xtcgaming.co.za/index.php/news/gaming/defrag/item/187-defrag-world-cup-2012

If I view source on the broken page, all header info is missing, only the item html is present.

Please advise soonest

Hi. There is an error that is breaking your page. Set error reporting to maximum through Joomla! configuration to see the error. Also note that Joomla! 1.7 is outdated and will not be supported by the next release of K2. You should update to Joomla! 2.5.