IBM OS/2 warp is still a very viable solution for always on terminals and is more common than you may realize. While not officially supported anymore, for a price IBM will still support it. While I wouldn't choose it for a new solution, I wouldn't run out and create new ATM software, if it is working well, just because of the OS. Would you be more comfortable if they were running windows?
It's been rebranded ecomstation and it's sold through another vendor now.
I was actually more surprised to see a reboot on a customer-facing ATM screen. I don't think I've seen that in the US. Maybe I lead a sheltered life.
Or maybe you just don't frequent ATMs a lot. I've definitely seen OS/2 Warp reboot screens on ATMs in the US.
Same name, but different banks. I don't think Santander (the Spanish bank) changed all systems of Santander UK (formerly known as Abbey) so your experience with a Santander ATM in Barcelona is probably irrelevant.
EDIT: typo
I was wrong. It looks like they share code in their on line services.