Known issues (Not that MG seem to care much).
This can happen if you have had earlier cards registered and then asked for them to be removed. Often, they are removed from the drop-down list, but are on file as registered at that casino. This can lead to the error of max cards even with none seemingly available. The casinos themselves can set the max number of cards and the transaction limits on individual accounts. It is possible for casinos to thus ban some players from using cards by setting the max limit to zero. This will also give the same error.
Also possible is that registered card limits are applied across an entire group of casinos operated by one entity. Weekly limits and transaction limits are shared between all casinos. This is the case at Casino Action, where a $200 deposit at one will reduce the card weekly limit (for that card) at all the rest.
Despite what it says, difficulty will be encountered with certain card types, such as Mastercard. VISA should be fine though.
Another minor bug that can abort the registration process is the cardholder name field. The true cause of the error is doing what the guidance tells you to do! It states enter the name "EXACTLY as appears on the card", unfortunately, the system will reject out of hand certain formats that card companies actually print on the card itself, and the error message is no indication. By trial and error I have found that one problem is caused if the card company prints a "formal" name on the card; such as Mr J Bloggs. The software will reject this, but actually wants you to enter "J Bloggs" (which is NOT "EXACTLY as it appears on the card"). Other than this, the software will accept middle initial, names etc on the card even if not matching the name registered at the casino - which can allow a card not belonging to the player past the process, only to be "shafted" later when the casino objects to the withdrawal on card mismatch grounds.
The first step would be to contact support and ask what the maximum card limit is. If this is not zero, ask if you have any cards on record at the group already that can be made available on the account. If the answer is none, you can then ask them to explain the problem with registering your specific card. The first two questions should narrow the ability for CS to Bullsh*t, and they will have to admit to either deliberately blocking you (or that particular card) specifically, or will have to guide you step by step through the process that they have established should be working normally.
I expect there are other bugs, and these probably relate to non-standard card formats not considered when the software was written, such as the 18 digit debit cards that the UK Internet bank "First Direct" use. The digits make it look more like a loyalty card from a supermarket rather than a bank card!
