https://github.com/NGSolve/netgen/pull/221

Index: ng/ngappinit.cpp
--- ng/ngappinit.cpp.orig
+++ ng/ngappinit.cpp
@@ -256,6 +256,7 @@ int main(int argc, char ** argv)
       Tk_MainLoop();
       Tcl_DeleteInterp (myinterp); 
 #ifdef NETGEN_PYTHON
+      py::scoped_interpreter guard{};
       py::gil_scoped_acquire ensure_gil;
 #endif
 
