OAuth2 SSO not working

SSO was working fine. But today, all reports using DirectQuery data sources to Azure SQL DB throw the error below when trying to configure the data source connections using OAuth2 and SSO (i.e. ticking the checkbox for "Report viewers can only access this data source with their own Power BI identities using DirectQuery.") I don't even the get popup to sign in after clicking the sign-in button.

I've tried with different datasets, new and existing datasets, against different azure sql dbs in different browsers.

I can connect fine from PBI Desktop (Version: 2.90.782.0 64-bit (February 2021)), SSMS and the Azure Portal.

If I untick the SSO box, it asks me to sign in and it works. But I need SSO.

Basic auth instead of OAuth has the same issue - it doesn't work with the SSO tickbox ticked.

Any one else getting this problem, or able to replicate it? 

Any idea on a fix, or is it an Azure fault?

Error:

The credentials you provided for the data source are invalid. Please check the credentials for all the data sources.

Status code: 500

Service version: 13.0.15430.58
Client version: 2102.3.04950-train
Cluster URI: https://wabi-uk-south-redirect.analysis.windows.net/