A lot of people in Germany have neither Visa nor Mastercard debit cards but use bank-issued Maestro debit cards (formerly known as EuroCards). They wont work with SE's payment system.
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Is it those cards you pre-load with a sum of money? We used to have Maestro cards but replaced those many years ago with regular Mastercard/Visa card with some limitations if you're under 18.
That type of card generally don't work on any online purchase. It's basically a ATM card (at least how they worked in my teenage years which is a long long time ago)
That just sounds like a typical Electron/Maesto card to me. I can't speak for all countries in Europe or EU alone but they are being phased out if not already in most places.
More common nowadays is what i explained earlier with Mastercard or Visa, or a restricted version for people under age of 18. Those card still works for online purchases.
These are the payment options I have and I just use ELV:
https://i.imgur.com/saqBqu8.png
My card looks something like that. It's just a normal debit card (or girocard).
https://sendesignal-hosting.de/alzey.../bankcard2.png
I am using crysta cause stupid I2501. The point is, there is no warning when you add crysta. On the entire SE website, there is only one page telling such information. And that page, it’s pretty hard to dig out.