Then I would suspect a missing dependency. You might try using Process Monitor to see what the application is trying to access when it fails. Another useful tool specific to assembly loading errors is the Assembly Binding Log Viewer.
One final thing you might want to look at is adding an .Net exception logger to the application. The issue with CCWs and late binding clients like PowerBuilder is that the information about any .Net exceptions that occur is that the exception information is lost by the time a late binding application gets a chance to ask for the details. A .net exception logger ensures you can still get that information, so you can see what went wrong. The following article explains how to do it, though it's written from an ASP perspective because ASP has the same issue.
.NET Exception Logging From COM Clients Without Modifying Code