Can't connect to bank account - Oops message

I’m trying to connect the Curve app with Santander through the Plaid connection. I tried to do it last year but didn’t work, so left it. IT still has the same problem.

When it tries to send me to Santander I get a Santander “Oops go back” message.

I could phone them, but I wanted to try here first as I feel they will tell me that it’s Curve’s problem to solve.

Any ideas?