Are you sure you have index.html defined as one of the Default Documents in your IIS configuration? It did not used to be there by default (iisstart.htm and Default.htm were defined) so you have to add it yourself.
Edit: Glad it helped sort out the problem. For your info, here is where Default Documents are defined in my version of IIS:
In our version of IIS 8 the Default Documents module was not available upon installation. After adding this module in IIS the problem seems to be solved! Thanks for helping!