Probably many of you encountered this error when invoking an orchestration exposed as a web service. Many of you knows how to fix this (see it here).
But the problem comes when the BizTalk engine is installed in a Windows XP machine, just like me.
Well, the solution is about the same. We need to add the application pool user to the BizTalk Isolated Host Users group. In Win XP that user is the ASPNET. So, add <machine_name>\ASPNET to that group and the problem is solved :)