Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
The mixing of men and women is a very serious matter and its mischief is very high. Lectures of knowledge and circles are not an excuse for alien people to mix.
The wives of the companions, may Allah be pleased with them all, did not attend circles or lectures of knowledge along with the companions in a single place where men and women mixed together, despite the fact that the first Muslim generation was very pious, chaste and righteous.
The wives of the companions were in need of hearing the Prophet (Sallallahu Alaihi wa Sallam); that's why they asked him to give them a specific day for them only. Al-Bukhari reported in his Sahih that Abu Said Al-Khudri, may Allah be pleased with him, said that the women said to the Prophet (Sallallahu Alaihi wa Sallam): "The men took all your time, O, Prophet of Allah! Reserve for us a specific day (to teach us). So the Prophet (Sallallahu Alaihi wa Sallam) promised them one day on which they would meet him; he admonished them and ordered them." Therefore, the Prophet did not order them to attend the circles of knowledge with the companions, and if there was any good in that they would have been the first to do it, as they were more pious and more righteous than the present generation.
So how could this happen in this era in which the hearts of the people are sick, and adultery, fornication and other disobediences have become widespread? It is more appropriate to avoid this mixing because it is forbidden. The correct opinion, without doubt in this matter is to stay away from any mixing of men and women, and to organize separate circles for men, and others for women, or it could be one circle but there should be a veil (curtain) that separates men from women. Do not take into consideration the statement of this female advisor who wants the mixing of males and females in this circle, and never seek her advice again. If you want to appoint a knowledgeable and pious person/as advisor for this association, whether a male or a female, then this is more appropriate.
As regards to reason the people stopped coming to the circles, we cannot determine that, as there could be many possible explanations.
Allah knows best.