Hi, Jörg--
I don't know if I have any answers, but having struggled with a few IUP issues, I thought I'd take a look. However, I tried to compile your code and ...
> Warning: reference to possibly unbound identifier `gridbox'
Yes, indeed it is unbound. There is no reference to 'gridbox' in the IUP egg (v1.6.0) source code. What version of the egg are you using - or did you add gridbox support yourself?