We have this installed on 1.21.1 serverside, we can take an enchantment off of an item once. Then, using different grindstones, different items, different books, the player cannot do it again. You can still see the enchanted book, but when trying to remove it from the grindstone, it simply snaps back in and the action can't be completed.

Are you sure that you have enough XP levels to disenchant another time? Disenchanting is fairly expensive, and since you mentioned it being installed server-side (and due to the fallback server-side cost display being broken at the moment), I take it you unfortunately can't see the level cost.
Does it work in creative mode?

I had enough levels, what's interesting is it just worked again for me in survival. It may have been a levels issue, I've had one book take 8 levels and another take 30, so who knows. I didn't know any display was broken