Memebrship Question

I think you should be fine with the captcha plugin. I’m not familiar with the S2 plugin but did a search and found an older post which may have some good info for you:

https://wordpress.org/support/topic/wordfence-blocking-access-to-php-file?replies=3

Finally, when using security plugins don’t enable the same feature on multiple plugins. For example if you have two plugins that have a firewall, only enable one plugins firewall.

-Brian