English question, please help?

Unless there were many beautiful scenes, you have use the first sentence: The scene was one of indescribable beauty. Alternatively, you could say: The scene was indescribably beautiful.

The scene was one of the indescribable beauties - This is Correct!

Scene is singular and beauties are plural.

Its a One out of Many construction of the sentence.

2nd sentence is correct:

"The scene was one of the indescribable beauties."

The phrase "one of the" in the sentence proves that there isn't only one indescribable beauty, it means there are many. The "scene" was described as "one of the.."
The first sentence would sound wrong.

2nd one is correct

Both are fine but mean different things.

The first says the scene was an example of indescribable beauty. The second is awkward, because it classifies a group of things (the indescribable beauties) and says it's one of them.

The second works better with a finishing preposition...

The scene was one of the indescribable beauties of the world.