In IIS 7.5 on Windows webserver 2008 I get a 500 response on any request. Checking in firebug if I'm being forwarded but the request is made and no forwarding, just get 500 response with a page that looks like Symfony response page.

Disabled all url re writes and requested index.html (still get 500) checked error pages under featured settings and detailed errors is checked. Checked the php error log and no writes have been made for several days.

I think someone has fiddled with the IIS server settings and got it to return 500 for any request (even locally) but I'm not able to find what it is.

[UPDATE]

While I was writing this and listing things I've tried I tried selecting detailed error messages at the root node, not the site node. Looks like some idiot changed file permissions so that IIS can no longer read the config file.