How can I get past invalid authentication headers error in IIS 7.5?

I am running a local website as one of several under Default Web Site in Sites in IIS 7.5. I am running Windows 7.The site was working fine for a long time and then suddenly stopped working this morning. I don't know why.

I was initially getting an error saying that my user id did not exist, although it seems to in the security tab for my code root. Now I'm getting this error.

" HTTP Error 401.2 - Unauthorized" You are not authorized to view this page due to invalid authentication headers."

with this error code: 0x80070005

I have windows authentication enabled and all of the others disabled at both the Default Web Site and the sub-site level.

Within Windows Authentication, I have the following two providers: NTLM and Negotiate in that order.

Looking in the properties for my site, all of the relevant accounts seem to be present and have the relevant permissions.

I've been trying to figure this out all day and I'm stumped. Any suggestions would be appreciated.