Annoula,
Sorry, but the forum you linked to mentions flash.gr as its original source... hardly a source of orthodox theology... as if our Catholic friends quoted CNN, Fox News or maybe RAI Uno as a source of Catholic theology.
There's an awful lot of Orthodox people who do believe the Theotokos was sinless, including several church fathers. Saint Silouan the Athonite (20th century) readily comes to mind (I had some exposure to his writtings a few years ago, so that's why he is the first that pops in mind). We don't make a dogma out of it though & I am aware there are also some other (fewer in number) Orthodox people who don't hold to that too strongly.
She was from birth to death fully human, nothing more and nothing less. She did make human
mistakes. For example, her "scolding" her Son when as a 12-year-old He disappeared from her and Joseph & preached in the Temple, that was "bad judgement" on her behalf. Understandably so, because as a human, she really couldn't completely fathom the Miracle that had happened to her & as a Mother, well, she was naturally worried what had happened to her Son. But as for sin, I repeat an awful lot of saints believed she didn't sin & please don't be shocked at that idea.
When we pray to her in the Small Compline (which I should be reading instead of writing this

), we call her Άσπιλε, Αμόλυντε, Άφθορε, Άχραντε, don't we. Sure we don't explicitly say Αναμάρτητε but but but... you get my point don't you... what else is there to say more than what we say already?