I've spent quite a while on this - the problem is with joomla core, I think you joomla installation is corrupted in some way. I've tried many things to get back bu all have failed. Therefore, I really think that you need to ask this question on the joomla.org forum - I am sure they will be able to come up with a solution for you, I suspect that this will involve reinstating some core joomla files.
Regards, Mark.
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