Yes, it works with a small number.Did you try with a small number, e.g. 20, 50, 100?
I have a playlist with just over 500 items, and it works with that.
I notice the program chunks requests into blocks of 1000, so I assume you would need somewhere near 1000 items to trigger the problem, but you probably know far better than I.
I have just built a playlist with 1666 items, and that also exhibits the problem.
I have also tried reducing the cluster size in ManagePlaylistModel.cs:329 to 100, but that doesn't help.
PlayItemsModel.CheckQueryPlayAction(() => CollectionUtils.Cluster(playlistData.MediaItemIds, 100).SelectMany(itemIds =>
cd.LoadCustomPlaylistAsync(itemIds, necessaryMIATypes, optionalMIATypes).Result), avType.Value);