I am having the exact same problem.
I have a Photosynth account that I've had for years (and I ONLY ever access it via a web browser or iOS app). I only have one Windows/Microsoft ID. A few months ago I tried to login in to Photosynth (via MS login) and was taken to the create new account page. My account is still active, so I can't use that username, but I can't access it either.
I've followed all help procedures that are available and after - literally - weeks of pursuing this, they told me to post here.
No "Forgot User Name" or "Reset Password" function? If we had direct access to our own accounts this could get solved. Kind of like every other internet account of anything anywhere.
So yeah, sorry I'm no help. Just wanted to let you know you're not the only one.