Value cannot be null error when accessing Administration pag

I get the following error when I attempt to access the CMS side of my website:
Value cannot be null.

This only happens on the Sitefinity side of my  site. IIS works fine (I get a permissions error when navigating to my.sites.ip.address/Sitefinity/)

I've also attached the error message to this submission in addition to the imgur link.

Found it.
Somehow my web.config got changed, the issue was with: <authentication mode="Windows" />, which needs to be <authentication mode="None" />. I luckily had a bitbucket repo (about a month old though) that had a slightly older version of my site. Did a diff compare and changed that discrepancy.