Unable to sign in to library account on Windows app (Northumberland Library)

Since the start of the week, I haven't been able to log into the Windows app for Northumberland libraries. I have an iOS account  with their library card which continues to work fine.

On the Windows app, the 'Sign in' screen shows the icons to sign in with Facebook, Twitter, Googgle and what used to be the Libaries (yellow circle with library card). On clicking the others, I'm taken to valid-looking option screens. On clickingthe libraries icon, I'm taken to a blank white screen titled Sign In. This has nothing to click eg to select from a list of libraries.  

I can login to the Windows app with a personal account (based on the email used in this post).

Windows 11 Home 23H2. OS build 22631.3593.

Dell Inspiron PC i5-10400, 40GB RAM.

When the sign in stopped working, I removed the Windows app and re-installed it from the Windows store. When this also just gave a blank screen white Sign in screen on clicking the the Library login icon, I again removed the app and reinstalled this time from the PressReader website - same again (and looked to give the same installation).

I get the same result on an HP laptop (also running Windows 11 Home 23H2).