Hi Gerroald and David,
I have been experiencing the same conflict with WishList Member and iThemes Security. This thread was just what I needed. To fully fix my problem, I needed to uncheck both ‘Filter Suspicious Query Strings In URL’ and ‘Filter Long URL Strings’.
The first feature (when checked) was causing 403 errors. The second feature (when checked) caused a White Screen of Death with no error messages (even with WP_DEBUG
enabled). I noticed the URL request generated by WishList Member was really long.
Thanks for guiding me in the right direction,
Dharmaraj