Everytime I use checkboxes in my form, it is not submited

I trying to use a form in one of my pages (so the user can sign up for a course. The form works perfectly into I try to use a label with checkboxes.
After that, when I click in submit the form, it keeps trying forever (it didn’t show any error message).

This is how I use in the form:

Preferência de datas e horários (*)

[checkbox de20a29demaio use_label_element “De 20 a 29 de Maio (de segunda a sexta, das 8h às 12h)” “De 15 a 26 de Julho (de segunda a sexta, das 14h às 18h)”]