Who wrote Maseches Sofrim and when did he live. Is it accepted by everyone?
The Rashba in Brachos 34a writes that is one of the minor masechtes that were authored by the early sages. He reiterates in his commentary on Bava Basra 13b that it is traditionally accepted that it was authored by great Sages. But he does not give a clear indication of exactly when those sages lived.
The opinion that dates the masechta the furthest back is that of the Beis Dovid authored by Rabbi Yosef Doviv, the av beis din of Salonica (sec. 365) which states that it is one of seven masechtos composed in the days of the Tanaim. Shalsheles Hakabalah (p33b) and Seder Hadorois write that was composed two or three generations after Rabbi Yehuda Hanasi, who compiled the Mishnah.
The Ramban writes in his Rosh Hashanah Droshoh that it was no accessible to the Bavli and that is why the Geonim disregarded this particular masechta. He also adds that it is part of Talmud Yerushalmi.
However, the Rosh (Halachos Katanois, Hilchos Sefer Torah, sec.13) maintains that it was written in latter generations. Many Achronim agree, proving so from the fact that the masechte quotes from Rava, Rav Ashi And Rav Kahana in the final chapter. This proves that it was written at least after or during these sages times.
It is also interesting to note that Maseches Sofrim quotes from the Gemoro, not the other way round.