Just to clarify - QuickFile is registered with both the FCA and Open Banking, and we do support Cater Allen.
The login page itself is something that’s controlled by Cater Allen. Although I am aware in the early days of Open Banking, Cater Allen did operate 2 login systems, only one of which was supported through Open Banking, the other had to be migrated by them (more info here).
I don’t think this is an issue now, but I could be wrong. Ultimately, and unfortunately, the error you’re seeing is on Cater Allen’s side, which is something we don’t have access to or control over (for security).