The TinyMCE control works fine in other places, e.g. the user profile and submission process. I suspect the problem is related to your URL rewriting configuration. I would suggest disabling that to see if it fixes the issue, and if it does, reviewing it to see if there is some resource that can’t be fetched in the administration area. I’m not able to test that myself, so I can’t give more specific guidance.
Regards,
Alec Smecher
Public Knowledge Project Team