Brilliant! Ted's solution worked for me too!
The interesting thing for me was that I upgraded from 6.x.x to 7.1.1 and it worked fine AND allowed me to login no problem after the upgrade. It was only after I'd logged out of my site and then come BACK to login again that the problem reared its head.
I also upgraded another site to 7.1.1 today, and that site is fine. When I checked web.config for that site, the "extra" bit (.Provider.AspNetProvider) isn't there, so SOMETHING must be causing the web.config file to be "changed".

I had this exact same problem today after upgrading from 7.0.6 to 7.1.2 - everything had been working fine since May 25 2013 when I did the upgrade to 7.0.6, which was still sql 2005. Then when I upgraded today I first had a failure because 7.1.6 *requires* sql 2008 or later, so I upgraded the db to 2012, then the upgrade went okay except *nobody* could log in. Found this solution and now logins work fine. I am a bit suspicious of what else I will find needs tweaking, but at least I am back to business.  BTW, unlike others who have come here, I truly love the support that DNN gets from the community and the fact that so many users of this particular CMS shake out the bugs within a fairly short time.  Other products, commercial or free, suffer from issues just as daunting but they don't have the community of support that DNN enjoys.  For me, the problems I encountered were fairly stress free since I had done my backups and squirreled them away in a safe place so I could have reverted if worse came to worse.  Proper preparation allows you to calmly and clearly address issues without having to install sweat wipers on your rose colored goggles.  Corny, but always worthwhile.

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Gene W

Thank you, Ted!!

Ted's solution worked for me too!