@David-Healey That's what I initially suspected too. But no I haven't changed architectures.
Edit: Actually it seems to do with my authorization/licensing system, it's triggering HISE's activate system. I left the RSA.xml in the project folder so just removing it clears up the error.Solution here: https://forum.hise.audio/topic/12825/errors-when-exporting-to-vst3/3