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The substack, 'Dia', is a substack of the mainstack, so, according to the documentation, thè stack should open (which it does) and not throw up an error message.
I open substacks this way all the time with no issues whatsoever.
On the other hand, I often find that exceptions raised are often ether not entirely accurate or too vague to be of use, so the actual error report isn't always that helpful. For example if something is crashing with geometry manager, you get completely nonsense errors (or no error but it crashes).
Might there be an open/preOpen stack/card handler in the substack that is causing the error? Or something else in the substack?
To test it why don't you try creating a new empty substack and repeat the process? I'd wager you do not see this problem with a new empty substack.