Adding some debug information, the exception come from .local/share/weboob/modules/2.0/lcl/compat/weboob_browser_filters_standard.py line 86. The snippet is:

        try:
            v = Decimal(text)
            if self.sign:
                v *= self.sign(original_text)
           return v
        except InvalidOperation as e:
            return self.default_or_raise(NumberFormatError(e))
        else:
            if self.sign is not None:
                if callable(self.sign):
                    v *= self.sign(original_text)
                elif self.sign == '+':
                    return abs(v)
                elif self.sign == '-':
                    return -abs(v)
                else:
                    raise TypeError("'sign' should be a callable or a sign string")
            return v

And indeed the 3rd to 5th lines may raise an exception whereas the same code exists in the else section which properly take into account the case where self.sign is a string.

Commenting the following code seems to solve the problem:

        try:
            v = Decimal(text)
#            if self.sign:
#                v *= self.sign(original_text)
#           return v
        except InvalidOperation as e:
            return self.default_or_raise(NumberFormatError(e))