@tobante So the issue was a virtual destructor that I had defined. Both gcc & clang did not complain, but on Windows MSVC gave an error message. I guess that caused ABI problems. After removing said dtor it loads on all systems.
So... we have a deal with a very large distributor/developer to carry our plugin. As you may guess this is a very very big deal for us.
However.... they want us to use their authorisation system, which is a C++ library ( a .cpp file and a header). I have no idea where to begin with this. I am assuming I add these files into the AdditionalSourceCode folder but from there I have no idea how to get it callable from within HISE....
Any help for a complete C++ novice gratefully received.