"Sign into Github" results in Forbidden error in Azure Data Studio

When using the “sign into Github” feature of Azure Data Studio, the authorisation page immediately returns “Forbidden”:

Any ideas? This seems to have started happening since I upgraded my access token to the new format. It’s been working for years before that.

I have tried using a different browser profile, signing out of GitHub, sending the link to a trusted colleague (who gets exactly the same).

Unfortunately I can only put one picture per post, here’s the prior authorisation screen: