Membership 2 plugin-Pay securely w/ Credit Card, loops back to Pay Securely with Credit Card

0 - Unable to install plugin due to other WordPress errors blocking the 'Plugin' Option on the WordPress dashboard.

Steps to recreate loop problem are:

1 - Go to URL

2 - click 'subscribe' on menu

3 - click 'signup' on any one of the options

4 - That takes you to where you enter any name, user name, email, password, etc., then click 'register my account'.

5 - That takes you to where you click on the 'Signup' button.

6 - That takes you to where you click 'Pay Securely with Credit Card'

7 - That takes you where you click 'Complete Payment'

8 - This is where the loop begins, as it takes you back to and asks you to 'Pay Securely with Credit Card' again.....

Can you please assist?

Many thanks.