Save post – Forbidden (not every page)

Hello guys,

when I save a page in the backend, I get the Error “Forbidden – You don’t have permission to access /wp-admin/post.php on this server.”

But this error is only on a few pages.

I tried the following things:
– Deactivated every plugin
– Changed to the default theme
– Cleared cache
– Changed to a different permalink structure

But its not working – I am clueless…

P.S. My WordPress is running on an IIS Server 😉