Hi, I ran a new test and I see that your plugin pass the following URL to PayPal, overriding the one in your profile:

SetExpressCheckoutRequest:
returnurl = "https://www.reliefteaching.com/membership/executive-annual/?s2p-option&s2member_paypal_xco=s2member_..."

When your plugin calls SetExpressCheckout it set a different return URL than the one you set in your profile. You need to check with them how to pass in the SetExpressCheckout API request a different return URL, if  they allow that.