I'm not sure I understand the differences between the working setup and the non working one. I guess you're not calling the callable service in the working one (although you might be reusing part of it's model.
To debug this, you should use trace, which in the case of callable service means you'll need to activate the "trace all" mode which traces all server side processes. See the bottom of http://www.tersus.com/#Id=46 for details.
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