Products wont update! please help!!

Hello,
I am just about to put my site live, and went to update one of my products. All I did was try and change the quantity in stock and clicked preview, but instead of the preview, it took me back to home page of the site! Bizarre!!
I wondered if it was something I'd done, so I clicked into another product and didnt even try updating it, just clicked preview, and the same again!
I have to state though that this isnt happening with all of the products, just some!?
Anyone any ideas at all?
Please help me! I need to get my site live :S

Does anyone recon this could be to do with permissions? As I've noticed the on my define pages editor its mention a permissions issue.
Odd though that it only happens with some products...

Have you added any add-ons recently?

What happens if you try to edit and update a manufacturer?