Thank you for your help Sue,

I have published the form and then I started it with
options-forms-choose form - MyForm
This way it works fine! The Voting buttons are comming on all receivers
and even my event scripts in the form are fired!
And "inherited" in the receivers form. That's exacly what I wanted!

Before I have launched the form by doubleclicking on the Myform.oft
icon in the explorer.
This way no Voting buttons and no scripts are running. Maybe a security
issue.

However it seems that publishing is anyway the rigth way to do it.

Thanks for your support. The problem is fixed for me.