By the place where the segmentation fault occurs 'int callConv = sysFunc->callConv;', I would follow Jeremy's suggestion to recompile the whole library. It has the looks of a mismatch between source version and compiled version, that somehow got past the linker.
Let me know if it still fails after that, and I'll try to look into it further.
Actually a bit disappointing, because now I have to really dig into the reasons why my code works perfectly on Windows, but fails on amd64..
I will keep digging, and post any questions in a separate topic.
Thanks for your attention.