I’ve discovered what I think is a bug with Redis-cache and S2member when the option “No (send each user a password-setup link after registration; recommended for best security)” is selected for the creation of users using the s2member join form. When this option is set a customised password reset email is sent allowing new users to set their own password. When redis cache is enabled the password set link in the email always generates an InvalidKey error. Disabling the redis-cache plugin and the error goes away.
I’ve posted this in the redis-cache support forum as well.
https://wordpress.org/support/topic/redis-cache-s2member-error/