Hello @jefftblumenthal , Thanks for the confirmation.
Do you see any browser loaded for the authentication? when you run the sample code?
Please refer to this Github issue: https://github.com/MicrosoftDocs/azure-docs/issues/60633#issuecomment-675250661
Could you please try using the Azure.Identity v1.1.1 instead of the latest version "Azure.Identity v1.2.0" to get unblocked on this issue as a workaround! Please let us know if that works for you.
Update: 08/26/2020 For InteractiveBrowser issue fix.
We have now the NuGet package fixed in the latest Azure.Identity(1.2.2)
So please make use of the latest NuGet Package Azure.Identity(1.2.2)
Please let us know over here or comment on the Github Doc to get further help in this matter.