Sign-in buttons unresponsive on Windows client.

I have a user in our organisation whose Windows Zoom client is unresponsive when trying to log in.
I can press 'sign-in via Google', but the button does nothing. Selecting 'sign-in via SSO' allows me to enter our Zoom domain, but won't pressing Continue does nothing...

I have tried uninstalling the client and installing the latest version, but get the same results.
I have also tried installing from our .msi (based on a slightly older version), but same results....

I'm at a real loss... any ideas?