Site Index breaks when 'Member' logged in

Im getting a really weird error Ive never encountered before:

When a ‘member admin’ is logged in (with limited but usual member permissions, ie can publish and edit their own entries to all channels, can upload files etc etc, the homepage of the site breaks - the templates do not call correctly, and instead call incorrect templates (embeds etc). All other pages of the site are unaffected, ie all templates parse correctly. Ive ran debugger and it returns nothing. ‘Console’ in FF or Chrome also tells me nothing.

Im stumped why this is happening. Ive not as far as I am aware made any specific setting for logged in members to see any alternate homepage - Ive searched around to find that in the member config and can’t see anywhere you would do that. All the urls are correct that are calling template paths etc (I use Deeploy Helper to manage that). I am running a whole bunch of modules and plugins, but have never had this happen before with similar builds.

Im using EE 2.5.3, and this is a fresh build, not an updated site. I have online and localhost versions, and the problem occurs on both sites.

Any suggestions on what else to try are gratefully received!

UPDATE: have moved this to the other EE community forum as probably better suited to that area. Apologies for double posting. http://ellislab.com/forums/viewthread/231466/