Well I don’t have a solution yet but I DO see what is happening. What I can’t explain is why it is happening semi-randomly. For example I WAS able to update an admin options panel but only once!
The failures are occurring in the header data sent with the request. When they fail they are not being sent as XMLHttpRequest requests and, in fact, other header data – such as cookies for example – are not being sent either. Now this is getting into waters where my knowledge is pretty grey but why this should happen in some places and not others is a big puzzle.
SP exclusively uses jQuery for all AJAX requests – although as we mostly use the .load() function this is technically AHAH. For reasons unknown, these requests are sometimes being stripped. Other times being sent correctly.
I would assume – and remember this is all grey to me – that the likelihood is that these headers are getting stripped when they arrive at the server but who knows – there may be something operating on them that you have installed.
I think you need to firstly have a chat with your server team about this. It is also worth pointing out, of course, that MS IIS is not a WP favourite server but it should work!
I do have screen shots to back up what I have stated above and will happily send them to you if you need them to look at.
Nice looking site by the way. Very attractive.