This error can happen when the default browser on iOs 14.X is not set to Safari.
The bank has been notified of the issue. In the meantime, try the workaround below.
What to do next?
On your iPhone, go to Settings
Find "Safari"
3. Hit "Default Browser App" and temporarily select "Safari" as the default browser
4. Try to authenticate using your Santander app again.
You're free to change the default browser back to your preferred one after your authentication is successful.
Hope that helps!