gamerule logAdminCommands false does not suppress the following error message as produced by summoning an item with bad NBT data or when spawning items using a spawner. The output error message

Can reproduce message by running following command:
summon Item ~5 ~ ~ {}

This command will spam the console with this error message: "Item entity #### has no item?!"

With mob spawners, the error occurs only twice per successful spawn cycle.