In version 2.5.1, a bug was fixed in the item conversion process. I asked Minecraft to convert from DataVersion -1 to the DataVersion of Minecraft 1.13. However, this caused Minecraft to assume that the item is from soem Minecraft version of years ago. This caused corruption for items with a BlockEntity tag (I think only shulker boxes in vanilla). BetterEnderChest 2.5.1 converts the items from the format of Minecraft 1.12 to the format of Minecraft 1.13, so everything should now work correctly.

I do not expect many servers to already run Minecraft 1.13 in production. Still, I included a fix all shulker boxes that were corrupted by BetterEnderChest 2.5.0. It applies this fix when an Ender Chest is loaded from disk. So please put any corrupted shulker boxes back into the Ender Chest, close the chest, and open it again after 10 seconds to apply this fix.