it appears validation is always being applied to the checkbox form elements, i have logged this as a bug to fixed in a future release.
in the mean time, delete the following line to remove the validation:
var fieldset_group_AGB_Bedingungen_gelesen_verstanden_Spry = new Spry.Widget.ValidationCheckbox("fieldset_group_AGB_Bedingungen_gelesen_verstanden_Spry");