Problem signing in to MS Teams app on iOS

I have a user experiencing a problem signing in to the SM Teams app on iOS 13.4. Auth is with ADFS, and that goes fine (username, password okay), Duo push is fine (and shows success in the Duo auth logs), then MS Teams gives and error:
“Sign-in Error: Please connect to the internet and try again”. User is connected to the internet (as ADFS auth and Duo challenge actually happen), so that seems misleading.

Anyone seen this or have any insight?

Regards,
Andrew