Error message when using coupons

I get the following error when I try to purchase any membership level on my site:

"There was an unknown error encountered with your payment. Please contact the site administrator."

I am using as a payment processor. Currently the account at is set to test mode, and the mode in the settings of Membership is set to Live (it wouldn't work at all in Sandbox mode).

This issue is almost identical to this one which never got resolved: