Membership 2 Pro Registration & membership Doesn't Work

Membership 2 Pro registration & membership selection does not work.

* After completion of registration page, I am taken to memberships available.
* When I select a membership, I'm taken to the confirm membership selection page.
* After I confirm, I'm taken to the payment page (but I'm signing up for a free trial"
* After I click "signup" I'm taken to a "complete payment" page (but I'm signing up for a free trial!)
* After I click "complete payment" I'm taken back to the "confirm membership" page

That process will go on forever if I keep clicking.

Then, I receive the following email.

Hi "new signup's username",

This notice confirms that your password was changed on "my site."

If you did not change your password, please contact the Site Administrator at "my admin email address".

This email has been sent to "new signup's email address"

Regards,
All at "my site title"
"my site"

* I do NOT receive the WP username & reset password email.
* I do NOT receive the Signup - Completed free trial email.
* I am NOT added as a new member.
* I AM added as a WP user.

If I don't have [ms-membership-register-user] on the Register page, then I'm simply given a blank register page.

We are on a reliable host high-end VPS and have no other problems with other WP installations on other domains on our server.

* All plugins except Membership 2 Pro & WPMUDEV Dashboard are deactivated.
* I have tried deactivating M2 Pro and also deleting and fresh install.
* Default WP 2015 theme is active.
* Cloudflare cache purged.
* Cloudflare paused for site.
* Using a different browser to do test signup.
* Cache purged on test signup browser.

Everything worked fine until a month ago I deactivated WPMUDEV Dashboard along with other plugins to troubleshoot an issue. I didn't notice it at the time when I reactivated Dashboard, but we were automatically downgraded to Membership 2. Some time later I noticed the downgrade and upgraded. All seemed ok until I noticed Membership 2 Pro-related issues.

A major disappointment is that Gravity Forms User Registration no longer worked correctly. I have now given up hope of using that.

Since then, it's been weeks of fix one M2 thing, then another doesn't work, fix that, then a different issue pops up.

All help is deeply appreciated.
Don