Don’t know if this will help but I faced same issue while working with U16 apparently by just removing everything after localhost/umbraco/ you can get the backoffice to load. Don’t know if there is any permanent fix for it.

Yeah, that's what I've had to do, but reading one of the forums, apparently V16.3 will have a fix for this.

where did you find a forum post regarding this?

https://forum.umbraco.com/t/backoffice-login-process-freezes-with-loader-animation-and-oauth-complete-url/3316 this is the forum. I've not personally tried version 16.3 yet so I'm not too sure if it has been fixed, only going off of what others have said.