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I really don't know what to suggest. As I am sure you have realised neither of us here can really work out what is going on with your environment and it has to be environmental to some degree. I don't think we have ever been defeated by something like this before.

One last thing that might be worth investigating is to ask your host if they have imposed 'mod_security' on your server and, if they have, could they turn it off even if just for testing. This module is the bain of all developers as it can block urls it deems may be suspicious based on pattern matching. I doubt this is it but a really aggressive mod_security setup can cause all sorts of problems.