Custom fields "Unique Field ID" error [RESOLVED]

I saw this error listed a few times on the old forum:

http://www.s2member.com/forums/topic/custom-registration-field-not-added/

http://www.s2member.com/forums/topic/cant-add-custom-field-unique-id-exists/

I’d like to report the solution I found in case anyone else runs into this. The file wp-content\plugins\s2member\includes\jquery\.htaccess had this content:

allow from all

I changed it to this:

<IfModule authz_core_module>
	Require all granted
</IfModule>
<IfModule !authz_core_module>
	allow from all
</IfModule>

https://wordpress.org/plugins/s2member/