Are you sure it's not just that your card has both EC and Visa Electron/Mastercard Meastro or similar, and that it's the EC part they accept?
It's been a few years since I've been to Germany, but for comparison, in Norway - while it's nearly unheard of now - you also used to be able to find places that'd take "BankAxept" bank cards, which would be pretty much every domestic debit card, and is similar to EC Karten, but not Visa Electron/Mastercard Maestro debit cards.
If you had a domestic debit card, you'd almost certainly have a Visa/Maestro logo on it as well and so it'd be easy for people to assume that was what they were paying with.
Works fine for me, as I said, I can pay with MC debit almost everywhere. The bonus is no-fee foreign currency payments, just for the basic MC exchange rate. Back then I checked, and the only others that offered that were mobile-only banks.
But from all the comments, I’m starting to wonder if SumUp had some focus on Lübeck and Hamburg for other places to not have the huge advances in card payments of recent years.
It's been a few years since I've been to Germany, but for comparison, in Norway - while it's nearly unheard of now - you also used to be able to find places that'd take "BankAxept" bank cards, which would be pretty much every domestic debit card, and is similar to EC Karten, but not Visa Electron/Mastercard Maestro debit cards.
If you had a domestic debit card, you'd almost certainly have a Visa/Maestro logo on it as well and so it'd be easy for people to assume that was what they were paying with.