IIS7 not .html or .js files but serves .aspx

I'm trying to implement authentication into my website. I have managed to be correctly redirected to a login page when I try to access the secured website and I can login. However, the problem is that after login I can't access static files (such as .html, .css, .js, etc.), but I can access without any problems .aspx files.

I get error 500.0 with error code 0x80070542.

I have actually found very precise description of my problem here, but the solution doesn't work for me (I already have IIS_IUSRS in there).

Has anybody experienced a similar problem? If you need to see parts of my web.config, let me know and I will add them.

Thanks