I have a live application on AppExchange that has been working for many months.  It uses a Session to login.  A Salesforce Sales Engineer has it installed in a demo org of their's and he gets the error message INVALID_SESSION_ID: Invalid Session ID found in SessionHeader: Illegal Session when it makes the getUserInfo call.  I have been working with him and have access to his org and know that the initial Session connection is being made (I have seen it connect in my code).  However, once the code makes the getUserInfo call, it gives that error message above.

I don't know if there is anything special about demo orgs, but from what I can tell, it looks fine.  In fact, he used this same org to connect to my system several months ago.

This org is on NA1.  I have other DE and EE orgs on NA1 that work with no issues.  It's just this one.

I am using the latest and greatest PHP toolkit.

Thoughts?