You can get checkbox value by alternate way that is, Instead of passing variable value pass the data extension name in the URL. Then read the data extension name as parameter then lookup for checkbox value on your submit page. I am assuming your checkbox type is boolean so the code will be
set @Email = email to match for checkbox value true or false
set @DE= RequestParameter('Your Data Extenion Name Passed as parameter')
set @value = Lookup(@DE, "checkbox field name", "Email", @Email)
IF @value == True THEN
Your Code If True
ELSEIF @opt == False THEN
Your Code If False
ENDIF