I set up a website to use SqlMembershipProvider as written on this page.
I followed every step. I have the database, I modified the Web.config to use this provider,
I know this is an old post, but I found an additional answer... For others in the future with this problem, I found that my web.config file somehow had the following added to the bottom (not sure how). Once I commented the out, it worked fine :) Even though I had everything above in the file set properly, this one line caused me over an hour of headache...