Are there any Catholic sects which disagree with the sinlessness of Mary? As a protestant, the concept of the sinless state of Mary is one of the hardest aspects of Catholicism to swallow. The historical account of the adoption of this doctrine doesn't lend any credibility to the view, either.  It thus seems incredibly likely that there are a large number of Catholics who also take issue with this belief (not to mention other beliefs on which Catholics and protestants traditionally disagree--transubstantiation, etc).

So my question is:  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.

Mary's freedom from sin since the point of conception, which Catholics celebrate as The Immaculate Conception, was affirmed in a Papal Constitution by Pope Pius IX.


  In the Constitution Ineffabilis Deus of 8 December, 1854, Pius IX
  pronounced and defined that the Blessed Virgin Mary "in the first
  instance of her conception, by a singular privilege and grace granted
  by God, in view of the merits of Jesus Christ, the Saviour of the
  human race, was preserved exempt from all stain of original sin."


And Papal Constitutions are binding to all Catholics to whom they are directed:


  Papal Constitutions are ordinations issued by the Roman pontiffs and
  binding those for whom they are issued, whether they be for all the
  faithful or for special classes or individuals.


That said, you can expect that many self-identified Catholics and "Catholic" groups don't believe the doctrine. But, by virtue of the Papal Constitution, they're not "formally recognized by" or "in full communion with" the Catholic Church.

And the importance of the doctrine may be illustrated by the fact that the feast thereof is one of the Holy Days of Obligation in the US.