Gandalf knows that Sauron is back. This directly contradicts LotR. In fact, there's no reason Gandalf would let Bilbo keep the ring once he knew Sauron existed.
Actually this is exactly like in the books.
The Fellowship of the Ring, "The Council of Elrond", p. 261
'Some here will remember that many years ago I myself dared to pass the doors of the Necromancer in Dol Guldur, and secretly explored his ways, and found thus that our fears were true: he was none other than Sauron, our Enemy of old, at length taking shape and power again. Some, too, will remember also that Saruman dissuaded us from open deeds against him, and for long we watched him only. Yet at last, as his shadow grew, Saruman yielded, and the Council put forth its strength and drove the evil out of Mirkwood and that was in the very year of the finding of this Ring: a strange chance, if chance it was.
As for the ring, Gandalf did not know it was the One Ring.
Then for the last time the Council met; for now we learned that he was seeking ever more eagerly for the One. We feared then that he had some news of it that we knew nothing of. But Saruman said nay, and repeated what he had said to us before: that the One would never again be found in Middle-earth. (...) [Gandalf] sighed. `There I was at fault,' he said. `I was lulled by the words of Saruman the Wise; but I should have sought for the truth sooner, and our peril would now be less.'
He finally found an ancient scroll to test if it is the One Ring, because on the surface it looks like any other minor magical ring.
And then in my despair I thought again of a test that might make the finding of Gollum unneeded. The ring itself might tell if it were the One. The memory of words at the Council came back to me: words of Saruman, half-heeded at the time. I heard them now clearly in my heart.
` "The Nine, the Seven, and the Three," he said, "had each their proper gem. Not so the One. It was round and unadorned, as it were one of the lesser rings; but its maker set marks upon it that the skilled, maybe, could still see and read."
This is where it all starts in Fellowship of the Ring.