failed hotfix install recovery

I 'm running CF 9.0.2 on a Win 2008 R2 server. I recently installed the hotfix for APSB13-13, which seemed to go fine. The next day I can no longer get into the CF Admin site, though the other pages continue to work. The error message reads:

HTTP Error 500.0 - coldfusion.server.SecurityService.removeAuthenticated()V

The page cannot be displayed because an internal server error has occurred.

I have tried to back out the hotfix and even recover from backed up CFIDE and CF9 diretories but the error remains. Aside from a full re-install, anyone got any ideas what I shoudl try next?

Thanks.