[Server thread/WARN] [FML]: Attempted to add a EntityItem to the world with a invalid item at (19.63, 22.42, -832.54), this is most likely a config issue between you and the server. Please double check your configs

forge-1.12.2-14.23.4.2705-universal
spongeforge-1.12.2-2705-7.1.0-BETA-3136
RealisticItemDrops-1.2.12

There's an experimental dupe patch option in the config. Try disabling that if it's on.

Can confirm this fixes the issue. For anyone having the same problem set:

# A workaround for machines that check their drops before subtracting items. Disable if there are noticable side effects [default: true]
B:"Dupe Workaround"=false