I just got this same error after doing Instant purchases since August. I have a Master Card.
I emailed them. Then I just waited a few days and tried again and it took the same card back.
Still no idea why
Waited a day, and I was able to add and verify my MC this morning.
Rejected card from yesterday also means I lost my ability to wire money, so I'll have to get re-approved. I got approved earlier this year for use on their Exchange, but never used that feature.