Are you using SystemWebAdapters version 2.0.0 by chance? We just updated from 1.4.0 on our system and had the same issue. The redirect wasn't working. If we signed into our old app it worked fine but not from our .net 9 blazor server app. I just downgraded back to 1.4.0 for both the .net 4.8 mvc app and our .net 9 blazor server and the login started working again. I think there may be a bug in 2.0.0 or something that needs to change in the config that I wasn't able to find.