Hi Ari, thanks for your reply.
I'm ok for a Monday morning, I hope you are too.
(And apologies if I'm replying in the wrong place (a new post), I'm a bit new to this interweb in the sky malarkey....)
I don't have any other security plugins enabled. At the moment I've just got Membership, Appointments +, WP Client and Cool Clock (hosted by WP Engine).
It looks like it's the plugin itself - under "Settings" -> "Membership Pages" -> "Account", it asks me to enter a password for a new user, and there is a "password strength" indicator beneath.
I can sometimes get this password strength indicator to show "medium", but even then it won' accept -> "Please enter a strong password".
Will that be the plugin itself, or something else I have set?