I am not sure what you mean by 'It happend every time I change (rename labels) in body of this form'. Do you sometimes see the contact form and sometimes you don't?
If you never see it, it is most likely a matter of the settings in menu.ini.append.php. Add the identifier of your contact form class to the identifier list.
If that does not help please give a bit more info.
Greetings from Luxembourg
Claudia
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