I'm working on Odoo.sh V13.

I have added new fields to the signup form. 

When I try to sign up, it sometimes does not work, and  I  get  the  error:

Could not create a new account.

to debug, I edited this code in the method do_signup:

except (SignupError, AssertionError) as e:
    if request.env["res.users"].sudo().search(
            [("login", "=", qcontext.get("login").replace(' ', '').lower())]):
        qcontext["error"] = _("Another user is already registered using this email address.")
    else:
        _logger.error("%s", e)
        if SignupError:
            _logger.error("%s", SignupError)
        if AssertionError:
            _logger.error("%s", AssertionError)
        qcontext['error'] = _("Could not create a new account.")

odoo.addons.fl_auth_signup.controllers.main: Expecting value: line 1 column 1 (char 0)
odoo.addons.fl_auth_signup.controllers.main: class 'odoo.addons.auth_signup.models.res_partner.SignupError'
odoo.addons.fl_auth_signup.controllers.main: class 'AssertionError'

What are SignupError and AssertionError? 

And why do they both show at he same time?

And why does my code often works but sometimes not (with the same values in the form)?