As of Minecraft 1.7.8, player data such as inventories is now stored based on UUID, not player name. This prevents downgrading, and it also prevents converting between online and offline mode. Despite Mojang leaving plenty of warning (https://mojang.com/2014/04/minecraft-1-7-6/), some were left in the dark.
Never fear though, For we have a tool to downgrade. Simply get this jar:
Run it with: java -jar ProfileConverter.jar and then follow the instructions. All you need to enter is the path to your Minecraft server world, eg: /home/minecraft/lobby/world/
The tool will then do the rest. You can ignore errors such as : java.lang.RuntimeException: Duplicate name md_5, however please report any other ones.
As of Spigot #1404 we will attempt to recover this data as it is used, but if in doubt, following the instructions above is a sane course of action.
See IRC: http://www.spigotmc.org/pages/irc/ or make a thread on the forums if you need any further help.
If you wish to disable this check for any reason (ie: you run lots of servers), you may add the following Java flag:
or simply create a file named .update-lock