Having a similar issue here.  Use QB Checking via Green Dot for my ProAdvisor account...and have an Amex that I use for credit transactions.  Always keep at least $1k in the Green Dot Account.

Went to pay Amex (just under $400), declined, charged fees by Amex because of this error on QB/Green Dot's end.  Can't get ahold of anybody, Green Dot doesn't even recognize the card number on their website.