Coupon - testing - takes user back to 'create an account'

Frustrating. i've added a coupon to give 10% off. All seems fine. However, when I test it it goes in a loop and doesn't work. Let me explain.
1. I go to the registratrion page and click on the 'sign up' button which takes me here: /register/?action=registeruser&subscription=1

2. I 'create an account' as instructed: and click on xxx which takes me to /register/ and shows me the subscription details, price, paypal button and the opportunity to enter a coupon code. It says: Have a coupon code? So I enter my coupon code ECHO and click 'APPLY' and it takes me back to the original /register page (outlining the different subscription levels) and so I have to start the process again... (only I can't use the same username/email as that's already in use... so I try with a different one, enter the coupon code, and bam, back I go to the first 'register' page... in a loop, never getting to sign up at all.

Please advise as a matter of urgency as the coupon is for a promotion going in a newspaper early next week and I'm wanting it to boost subscriber numbers rather than frustrate potential members before they've even signed up. Thank you.

Kind regards
Cheryl

PS happy to provide a login if required via email.

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