Some users of an app of mine are reporting links dont open in the browser. I always launch them with shellexecute(0, 'open', 'http://...
what could I check for an incorrect(?) setting in the http link associations?
You're assuming that the browser registered the open verb. It may not have done so.
Just pass nil as the second parameter and omit the open, and let the OS determine what the default action is for the http:// protocol, and you should be fine.