Membership 2 Pro Plugin & PayPal Standard Gateway Not Working In Sandbox Mode

Hi - I am currently trying to test the PayPay integration with the Membership 2 Pro plugin.

Haven't been able to successfully sign up for a paid membership in sandbox mode. When I click Subscribe on the Membership Types page, I get taken to a PayPal page that displays the details of the membership I am signing up for. When I click the log in button and then log in with a PayPal Sandbox account, I get taken to the PayPal Account Dashboard and not back to the Membership page where my sign up should be confirmed. The transaction never goes through.

The PayPal Merchant Account ID has been set to the Sandbox email address and the IPN listening URL has been set in the sandbox seller account.

When I check the Transaction Logs under Billing, the user sign up line item states the following in the details: Ignored: Missing POST variables. Redirect to Home-URL. [Irrelevant IPN call]

Any idea what the issue could be? Support Access has been granted.