Are there any Catholic sects which disagree with the sinlessness of Mary?

the concept of the sinless state of Mary is one of the hardest aspects of Catholicism to swallow.

It's not just Catholicism that believes this, Lutheranism, the very base of Protestantism does as well.According to Sunni Islamic sources, this belief not only exists but the only 2 people who have not sinned in this world is Mary and her Son, who they deny is God. Lutherans, Anglicans, Catholics, Orthodox, Anabaptists and many other sects in between have never denied Mary's perpetual virginity.

The historical account of the adoption of this doctrine doesn't lend any credibility to the view, either.

I suppose that would depend on your churches following of Sacred Tradition. Without sacred tradition, it could have very well been a torture stake that Jesus was put on, instead of a [cross.][3]For Catholics and Orthodox alike, it's important to follow Sacred Tradition, as passed down from Church founders. For Catholics, Paul requires the aid of Tradition in 2 Thess. 2:15.

So then, brothers and sisters, stand firm and hold fast to the teachings we passed on to you, whether by word of mouth or by letter.

Concerning it being a doctrinal belief, this only means that it was affirmed by a Pope, not that Catholics now started believing in it.

It thus seems incredibly likely that there are a large number of Catholics who also take issue with this belief

I haven't met any, but where could they base this belief off of? Do you mean they doubt Sacred Tradition regarding Mary or that they doubt the doctrine that fell from the Sacred Tradition that was affirmed by the Pope?

Are there any sects, movements or other formally recognized groups (as opposed to dissenting individuals) within the Catholic church who do not believe in the sinlessness of Mary, and if so, how are they recognized and treated by the rest of the Catholic church?

No, one cannot be Catholic and think Mary is a sinner. All 23 churches in communion with Rome have a belief that Mary was born without sin, and stayed sinless. To not have this belief and identify as "Catholic" puts them at the same level as Protestants, that is to say, they have a some type of Communion, but not full. The same way how Converts from Protestantism do not have to get re-baptized if joining a Catholic Church, since their baptism carries over.

On the Perpetual virginity of Mary (original question).

The doctrine is part of the teaching of Catholicism and Anglo-Catholics, as well as Eastern and Oriental Orthodoxy, as expressed in their liturgies, in which they repeatedly refer to Mary as "ever virgin".

Source - Wikipedia, which cited vatican.va, items 499 + 500

499 The deepening of faith in the virginal motherhood led the Church to confess Mary's real and perpetual virginity even in the act of giving birth to the Son of God made man.154 In fact, Christ's birth "did not diminish his mother's virginal integrity but sanctified it." and so the liturgy of the Church celebrates Mary as Aeiparthenos (ἀειπάρθενος), the "Ever-virgin".

This suggests that it is widely a part of the catholic beliefs, and therefore there can not be an official sect that doesn't believe.

That would be similar to them believing that Mary was not a virgin before she bore Jesus (a fundamental part of the Christian faith). However, there may be one. I would expect them to not be accepted by the main church, at least the officials of it. I would have thought there there would be Catholics that disagree with this belief, but, as it is a main belief, they wouldn't necessarily be that open about it.

No; it is not possible to hold such a belief without being considered either heretical, or at least withdrawn from full communion with the Catholic Church.

On 29 June 1998, the Congregation for the Doctrine of the Faith issued a document which discussed what teachings of the Catholic Church must be held by all Catholics. Section 11 of this document provides a list (not exhaustive) of such teachings.

The document was a commentary on a "Profession of Faith" which the Church requires of all who exercise an official Church function—including pastors of churches, teachers of theology, and others. The teachings are grouped by the CDF under three headings, depending on which paragraph of the Profession they refer to (see sections 5, 6, and 10):

[Doctrines of the first paragraph] are contained in the Word of God, written or handed down, and defined with a solemn judgment as divinely revealed truths either by the Roman Pontiff when he speaks 'ex cathedra,' or by the College of Bishops gathered in council, or infallibly proposed for belief by the ordinary and universal Magisterium. ... These doctrines require the assent of theological faith by all members of the faithful. Thus, whoever obstinately places them in doubt or denies them falls under the censure of heresy, as indicated by the respective canons of the Codes of Canon Law.

[Doctrines of the second paragraph include] all those teachings belonging to the dogmatic or moral area, which are necessary for faithfully keeping and expounding the deposit of faith, even if they have not been proposed by the Magisterium of the Church as formally revealed. ... Whoever denies these truths would be in a position of rejecting a truth of Catholic doctrine and would therefore no longer be in full communion with the Catholic Church.

[Doctrines of the third paragraph include] all those teachings ­ on faith and morals - presented as true or at least as sure, even if they have not been defined with a solemn judgment or proposed as definitive by the ordinary and universal Magisterium. Such teachings are, however, an authentic expression of the ordinary Magisterium of the Roman Pontiff or of the College of Bishops and therefore require religious submission of will and intellect. ... A proposition contrary to these doctrines can be qualified as erroneous or, in the case of teachings of the prudential order, as rash or dangerous ...

According to the document,

To the truths of the first paragraph belong the articles of faith of the Creed, the various Christological dogmas and Marian dogmas

However, it does not specify precisely what those dogmas are. It refers, however, to the Denzinger-Schönmetzer Handbook of Creeds, Definitions, and Declarations on Matters of Faith and Morals, specifically to the sections regarding the papal declarations on the Immaculate Conception and the Assumption.

Thus, Catholics are certainly obliged to believe that Mary was free of original sin. I do not see a specific declaration that Mary was also free of personal sin; however, this belief appears to have been taken for granted. For example, the Apostolic Constitution Ineffabilis Deus, in which Pope Pius IX promulgated the belief in the Immaculate Conception, describes Mary as "absolutely free of all stain of sin"; and Pope Pius XII's encyclical letter Mystici Corporis Christi describes her (paragraph 110) as "free from all sin, original or personal".

Further, Thomas Aquinas discusses the question in the Summa Theologica (Third Part, Question 27, Article 4). He says that Mary

would not have been worthy to be the Mother of God, if she had ever sinned. First, because the honor of the parents reflects on the child, according to Prov. 17:6: "The glory of children are their fathers": and consequently, on the other hand, the Mother's shame would have reflected on her Son. Secondly, because of the singular affinity between her and Christ, who took flesh from her: and it is written ( 2 Cor. 6:15): "What concord hath Christ with Belial?" Thirdly, because of the singular manner in which the Son of God, who is the "Divine Wisdom" (1 Cor. 1:24) dwelt in her, not only in her soul but in her womb. And it is written (Wis. 1:4): "Wisdom will not enter into a malicious soul, nor dwell in a body subject to sins." We must therefore confess simply that the Blessed Virgin committed no actual sin, neither mortal nor venial.

If this is not a "doctrine of the first paragraph", then, it appears to be at least a "doctrine of the second paragraph", to be held by all Catholics.

I am unaware of any organized groups of self-proclaimed Catholics or former Catholics who have adopted such a belief corporately.