checkboxgrpoup , a bug ?

I have set up a checkboxgroup as below pics:

I tried to select all boxes to see if the database correclty shows.

As strange at it looks, the first selected option always is "0"

and the debugger:

 [einsteige_ort] => Array
        (
            [0] => 0
            [1] => Gebenstorf
            [2] => Trafo
            [3] => Schademuehle
            [4] => Eurobus
            [5] => Lauffohr
            [6] => Banhof
            [7] => X
        )

Why that and is it there a workaroud?

Hi kraadde

Most propably there is another field on the page with the same name and a value = 0, this may be a hidden field, I need to check the form page to verify that

I could send you a copy of the debugger if it helps, but there are confidential information included that i need to delete. Is that an option?

her is ab bakup