Here is some information about those errors from a programmer's view. I am not really defending SE, just giving the rest of you a little bit more information based on personal experience with these kinds of errors in general.

The errors that are being tossed out tells their computers that the bank is declining the card even though it is not. This makes the error really hard to track since mixed into people getting the error in the wrong, there are the cards that the banks are really declining. The invisible lock out is actually common to use in online payment processing because when it is working properly, it protects the legitimate holder of the card in the event that the card being used was reported lost or stolen.
The error seems to be fairly random with who will get an error and when. (For example, my husband just got a subscription three days ago, but my other friend is getting errors.)
I hope that just a little bit of extra information at least helps a teeny bit.