On the other day I was troubleshooting on the SharePoint Access denied error. The System account and Users who has full rights also got the access denied error.
We checked ULS and took couple of fiddler traces on the site, nothing actually pointed out to the right direction.
The above error can be for many reasons, below is the one of the reason, which can help
We then checked the "Site collection quotas and Locks" under Application management - SharePoint Site Management
We found "Read-Only" was selected
We then changed the settings to "NOT LOCKED"
Then did an IISRESET
We were able to browse the site successfully