TYPO3 Core

Yes of course. Here it is:

enctype = application/x-www-form-urlencoded
method = post
prefix = tx_form
confirmation = 
postProcessor {
    1 = mail
    1 {
        recipientEmail = daniel.kempf@webtech.ch
        senderEmail = info@dasbad.ch
        subject = Checkbox value prüfen
    }
}
10 = CHECKBOXGROUP
10 {
    class = fieldset-subgroup
    legend {
        value = Wählen Sie eine Farbe:
    }
    name = farbwahl
    10 = CHECKBOX
    10 {
        label {
            value = gelb
        }
    }
    20 = CHECKBOX
    20 {
        label {
            value = blau
        }
    }
    30 = CHECKBOX
    30 {
        label {
            value = rot
        }
    }
    40 = CHECKBOX
    40 {
        label {
            value = grün
        }
    }
}
20 = CHECKBOX
20 {
    name = abc
    label {
        value = Abc
    }
}
30 = SUBMIT
30 {
    name = 6
    value = Submit form
}

Could you please test this form?

I did a small cleanup and added the value attribute itself. Your form had just label.value defined (which is for the label). Please test this form. I have also enabled the confirmation page to test the form.

enctype = application/x-www-form-urlencoded
method = post
prefix = tx_form
confirmation = 1
postProcessor {
    1 = mail
    1 {
        recipientEmail = your@mail.com
        senderEmail = your@mail.com
        subject = Checkbox value prüfen
    }
}
10 = CHECKBOXGROUP
10 {
    class = fieldset-subgroup
    legend = Wählen Sie eine Farbe:
    name = farbwahl

    10 = CHECKBOX
    10 {
        label = gelb
        value = gelb
    }
    20 = CHECKBOX
    20 {
        label = blau
        value = blau
    }
    30 = CHECKBOX
    30 {
        label = rot
        value = rot
    }
    40 = CHECKBOX
    40 {
        label = grün
        value = grün
    }
}
20 = CHECKBOX
20 {
    name = abc
    label = Abc
    value = Abc
}
30 = SUBMIT
30 {
    name = submit
    value = Submit form
}

With your code I get following Mail:

Wählen Sie eine Farbe:
gelb    gelb
blau    blau
rot    rot
grün    grün

Abc    Abc

As soon as I use the form wizard and do some changes and save it, then it breaks the code to an undefined code:

enctype = application/x-www-form-urlencoded
method = post
prefix = tx_form
confirmation = 
postProcessor {
    1 = mail
    1 {
        recipientEmail = daniel.kempf@webtech.ch
        senderEmail = info@dasbad.ch
        subject = Checkbox value prüfen
    }
}
10 = CHECKBOXGROUP
10 {
    class = fieldset-subgroup
    legend {
        value = Wählen Sie eine Farbe:
    }
    name = farbwahl
    10 = CHECKBOX
    10 {
    }
    20 = CHECKBOX
    20 {
    }
    30 = CHECKBOX
    30 {
    }
    40 = CHECKBOX
    40 {
    }
}
20 = CHECKBOX
20 {
    name = abc
    value = Abc
    label {
        value = Abc
    }
}
30 = SUBMIT
30 {
    name = submit
    value = Submit form
}

The form wizard should not be used after manually changing the form. The wizard in 6.2 does not support writing the value attribute of the CHECKBOX element. This is a separate issue and should not be handled here. It would be very nice of you creating a ticket for this problem. Thx!

So besides this issue the checkboxes are now transferred as expected but you are now having the problem with the layout of the email (I guess). Please check the massively updated documentation of EXT:form regarding .layout: https://docs.typo3.org/typo3cms/extensions/form/0.3/Configuration/Layout/Index.html#change-the-layout-for-a-specific-view.

This documentation is a first draft and neither final nor merged yet. The documentation shows how to change the layout for a specific view. That way you could change the general layout of the CHECKBOX element within the email view. Try it out. I recommend not using the general .layout functionality. It is much better to address the 3 views separately.

   ...
   # customize postProcessor/ mail
   postProcessor {
      layout {
         checkbox (
            <td><div class="textline"><inputvalue /></div></td>
         )
      }
   }
   ...

Closing that issue is ok. It would be very kind if you will open a new issue for the checkbox-wizard issue.

Yes, did it already. See #72308.

Great! Thank you very much.