After a ton of investigating to track down the problem, I found it has to do with IE and not with Slacker. This is on windows mobile 6.5.
In IE (Internet Explorer) browser on the phone, go into tools, privacy and clear the history, cookies, cache, etc. and slacker should work again.
I'm posting this in case anyone else runs across this issue. It was hard to figure because no one ever uses the IE browser that came on the phone and no one would think that the Slacker application made use of it. Heck, few enough people even still use Winmo as it is.