using Client side validation, you have a number of different types of validation you can apply, one of those validation types is a regular expression. another validation type is required / not blank. You should not be using regular expression validation for the checkbox. You should remove that validation type and replace it with required / not blank validation.
if you go to Insert -> WebAssist -> Validation Toolkit -> Client Validation you will restart the validation toolkit wizard where you can change the validation that was applied.