Hi,
just to confirm: this really seems to be account related. I am facing the same issue as StarLu. I did get Workspaces to work a few weeks ago, and today I wanted to get to them on another computer, which didn't work (same 404 error as StarLu). They continue to work fine on the first one, even after signing out and back in. I tried to isolate the cause (trying Edge on a Mac, on an fresh Win11 install in a VM, with the Canary version...) The only thing that worked (but is obviously not a solution) is to create a new Microsoft Account. My account is pretty old (a @msn.com from the early 2000's), but I don't think that should matter.
I did send a feedback report, hopefully it can be resolved, Workspaces is a nice feature.