Your very same problem has been reported by 4 additional users. We have performed tests with VISA and MasterCard and everything is fine, we could not manage to reproduce the problem in any way, unfortunately. We also continue receiving payments regularly.
Therefore the problem is puzzling, to say the least. Our reports to PayPal customer service are ineffective, because in almost every case a payment through a credit card works swiftly (at least with VISA and MasterCard, both credit cards and prepaid cards). It might be related to some credit cards only, or some PayPal malfunctioning, or some increase of PayPal risk management system threshold - although in this case an error message would be expected (all the credit cards transactions are completely handled by PayPal servers, we never come to know credit cards data). We should be able to provide a new credit card payment option with a different payment processor in the very near future.
At the moment, we also accept Liberty Reserve and Bitcoin, if this may help you.
Also, have a look at http://www.tipandtrick.net/credit-card-cannot-be-used-for-paypal-payment-error
It's not the same issue, because in the reported cases no error message is returned, but it might provide some helpful hints anyway.