Because htoa_plugins.dll is a plugin for Houdini that uses the Houdini api.
maya loads a digital asset which may be autonomous (don't need mtoa within maya) or it must be ready to calculations with mtoa. if no, then it's a bug.
there is a well known difference between apps because of their node structures, but i've never experienced such a mess with arnold versions before. i can make a new shader in maya instead of exported from houdini. so why there is no option to include|exclude htoa from DA?
Actually, that's a Houdini DSO Error message box. So it's not Maya, it's a Houdini/Houdini Engine/HtoA mismatch.
i see, it appears when maya starts, when "she" loads "his" engine ) well, i hope it's not too complicated to fix. thanks for your answers Stephen