Spotify - returns with no error but the user isn't auth'd

[scgough]

Hi
I'm using your Spotify auth provider and everything is working nicely most of the time. I do have a strange seemingly intermittent issue on my live site though and I can't figure out why/what is happening.

Every so often the user authenticates and is returned to my site but they are not logged in.
There are no exceptions and the user ends on the site homepage with the querystring ?_=_

I can't tell if this is something server side or something in the auth code as it always works when I run it via Visual Studio in debug.

Has anyone else experienced something like this?