I found out why the OnClosWindow-event didn’t work. It’s not a problem of the operating system, it’s a problem of the formativ settings. When I enable the command blocker option in the system settings the event occurs. When I disable the option the event doesn’t occur.
This is what I found out.
By the way, in the Formativ Creator Users Guide also stands, that disabling the command blocker will stop many Groupwise event notifications from being received by Formativ.
Thanks for your help anyway.