Clicking the Signup buttons on the Memberships sign-up page simply go nowhere (the Memberships page simply reloads), but I can't see any relevant errors in wp-content/debug.log (WP_DEBUG etc are set true) or figure out what's going on, what should happen next or how to diagnose the problem any further.
Relevant additional information:
• M2P v18.104.22.168 on network activated on WPMS 4.2.2.
• Network-wide protection is enabled (ie MS_PROTECT_NETWORK defined true)
• Multiple memberships add-on is enabled (makes no difference if this is switched off)
• Logged in as a basic user WordPress user
• PayPal Standard gateway in sandbox mode enabled (as only payment gateway)
• This is after an upgrade from M1P that was working okay (more or less, though upgrading a membership was a bit broken, but mainly because of PayPal API restrictions not because of a bug in M1P)
We have SSL, but it doesn't seem to make any difference whether the page is accessed over https:// or not.
Nothing obviously relevant appears in the browser's dev console, and I can see the POST request in the network activity log with form data that looks sane (nonce, membership_id, action=membership_signup, step=payment_table). It just doesn't do anything but reload the same page.
The Membership in question is set up for recurring payments and PayPal is enabled for this membership type, also set for public (or I guess it wouldn't show up), the level is set active and it is set as a paid membership level. I've not set up any protection rules yet (nor had I before I upgraded from M1P).
It's also worth noting that I have customised the Register and Memberships pages somewhat to force the user to log in (or create accounts) via the traditional login mechanism, mainly because I don't want users bypassing the security mechanisms that the stock login and signup pages have enabled, but also because I want users to have confirmed their email address before they sign up for any memberships.
This is on our staging site, on which I have just enabled Support Staff access.