I’m having the same issue but do not have the membership plugin.
The only change I made to my site was update the plugin – soon after that I had MANY visitors letting me know about this error and not being able to access the site.
It seemed to be happening mostly to mobile users and people who were clicking my links from a Facebook page or from twitter.
I softened the Login Security options to try and resolve the issue but it didn’t help.
The only way I could resolve the issue was to revert back to the previous version of the plugin.
