MP, best hand South.
I made a mistake. I forgot to mouse over my double before choosing it. It did not occur to me that it might mean anything other than penalties. It still doesn't (occur to me).
I did remember to mouse over the meaning that double would have meant on the previous round of bidding (over 2D), and it described it as biddable Diamonds with 15+ total points. I was still minded to double on the previous round. At least partner would more likely have stood it based on the explanation, but I was concerned that the auction might not die there, in which case I did not want partner to assume that my high card strength was better.
I have to acknowledge that we are somewhat in the realms of make-believe, my having asystemically passed partner's forcing 1S response, based on my privileged information that partner had at most 11 HCP. Were this the sole root cause of the problem then so be it. Mea Culpa. But in that case the subsequent calls' explanations should be blanked out, if GIB cannot cope with partner making impossible earlier calls. Instead it decides that my double of 3D is support-based. I think that 1S response by a passed hand would have been systemically non-forcing, probably showing precisely three trumps and a minimum. It would not be unreasonable to assume this anyway.