I just watched this film on YouTube; it's there for free in version with very good image quality. It was a box office flop, and has a poor reputation among classic film devotees. But if you enjoy musicals, you might want to give it a chance.
I mildly enjoyed it. Fairbanks Jr. Is excellent, giving a spirtited performance with the right touch of believable emotion and light tungue in cheek playfulness. The sets are opulant. Fredrick Holander's music is lovely, though the song lyrics (by Leo Robin?) are mediocre and banal. There are occasional playfully funny moments.
On the down side, the film overall lacks the charm, subtle sexiness, and light touch that Lubitch would have given it, if he had directed the entire thing, and his capabilities were not greatly diminished by very poor health. Grable is OK in the role, but far from the ideal actress-singer for it. I've read that Fox head Zanick wanted to broaden her appeal and put her in more high class roles, so she made three classy pictures and all of them, including this one flopped. (One of them is excellent, and I highly recommend it, "The Shocking Miss Pilgrim".)
That Lady in Ermine seems to have two endings; and I felt let down by the second one. It should have ended when the Fairbanks character rides away, bidding the Grable character a regretful goodby. But then there's a entirely unnecessary scene, which seems tacked on.