Heiko Hartwig Good question. Is SumUp actually working with girocards, i.e. are the transactions really handled via the girocard network, or do they use Maestro and V-Pay in the background? Because while their website lists the very old "EC"-Logo (which actually belongs to Mastercard, not the DKW), and also speaks about fees for "EC-Karten", in the support pages they never say anything about girocard acceptance.
So my theory is: They put the EC-Logo on their website and on their shop stickers in order to not confuse the average consumer who doesn't know anything about payment networks, but actually process transactions via Maestro and V-Pay.
As pretty much all German cards are co-branded with one of these, that would still just work fine as long as Chip+PIN is used. Maybe this model breaks down when contactless is used, where the card only includes girocard functionality.
Edit: Ok Roland confirmed what was my suspicion.