security check failed on login

My web site has multiple pages but no active blogs. If I don’t delete the cache in Super Cache daily users can’t get in and they receive “security check failed on login”. I am using e-member and multiple other plug ins. I would like to continue to use Super Cache to help with speed but looking for solution to blocking of the site when the cache fills. Thanks

http://wordpress.org/extend/plugins/wp-super-cache/