Here is the issue, as explained by Daily Pakistan. Under prior law, the nomination form that a candidate for office was required to complete included the following for all Muslim candidates:
I, the above mentioned candidate, solemnly swear that--
(i) I believe in the absolute and unqualified finality of the Prophethood of Muhammad (Peace be upon him), the last of the prophets and that I am not the follower of anyone who claims to be a Prophet in any sense of the word or of any description whatsoever after Prophet Muhammad (Peace be upon him), and that I do not recognize such a claimant to be Prophet or a religious reformer, nor do I belong to the Qadiani group or the Lahori group or call myself an Ahmadi.In the 2017 legislation, this statement became an unsworn declaration by the candidate. Hamid said that this was a clerical error, and earlier this month Pakistan's National Assembly amended the 2017 Bill to restore this anti-Ahmadi provision as a sworn statement in the candidate nomination form. (TheNation). That apparently did not assuage protesters' objections. AP reports that at least 6 people were killed and 200 wounded in clashes yesterday between police and protesters in Islamabad. Other opposition members of Parliament have different kinds of objections to other parts of the 2017 Bill.
UPDATE: Reuters (11/28) reports on the negotiated settlement of the conflict, though the military's role in the negotiations has raised questions.