Membership Pro 2 - toggles not working


On pages and for menus, I click on the access control toggles to remove access for certain member levels but I just get the spinning wait icon and nothing changes.

Please see:

I had previously installed the old membership plugin so I did the upgrade as per the instructions to Membership 2 Pro. Everything was fine until I ran into this issue.

Other things I have tried:
- made sure that WP core was latest
- disabled all other plugins - same effect
- updated all plugins - same effect
- deleted plugin and reinstalled - same effect

Thanks in advance for your help,