"Value cannot be null" Error on Sitefinity Backend

I randomly started receiving a "Value cannot be null exception" when trying to access the Sitefinity backend log-in screen (see attached screenshot).    I receive this error for both of the following cases: 
http://localhost:37499/site/Sitefinity/   (direct)
http://localhost:37499/site/Sitefinity/Authenticate/SWT?realm=...  (redirected from other page)

I can't pinpoint exactly when it started happening, but it was soon after I changed the permissions of a page to Authenticated users.  I have since reverted this change but am still experiencing the issue.  I now have to use the log-in page I created within my web site and log-in as an Administrator in order to do any backend administration.

My site is running on Sitefinity 5.1.3210.0 with IIS 7 (via Sitefinity Project Manager) and using Claims Authentication.

I have searched the forums and found the following thread which seems like the same issue I am experiencing. However, none of the suggested solutions seem relevant in my case.
http://www.sitefinity.com/devnet/forums/sitefinity/developing-with-sitefinity/can-t-log-in-in-the-cms-site.aspx