Thank you for the examples. I was trying to make use of the placeholders (like $T for example) which I completed at the end in the Connections tab.
Now I got it working by only creating the local parameters (HIDE_WINDOW_A, HIDE_WINDOW_B, ...) and giving them a default value '0' in the connections table.
Then in the button's and windows which are to be hidden I used these newly created local parameters directly. (instead of using the $T).
And now it is working fine. :-)... still don't know what I'm doing wrong when using the connections table....