Variadic functions of Intuition were always trouble makers for different versions of GCC compilers and m68k.
Instead of creating inline code use the amiga.lib provided code, which works flawlessly for Intuition/MUI/ReAction.
#include <proto/...>
#include <proto/...>
#if defined(__GNUC__) && !defined(__amigaos4__)
#define NO_INLINE_STDARG /* Take f.e. NewObject() from amiga.lib for gcc m68k */
#include <clib/intuition_protos.h>
#include <proto/intuition.h>
#undef NO_INLINE_STDARG
#else
#include <proto/intuition.h> /* Take/Create inline code f.e. for NewObject() */
#endif
#include <proto/...>
#include <proto/...>