Autorank console error log

[09:00:16] [Thread-41/INFO]: [Autorank] Could not find name of account with uuid: 'c1ef7694-fc67-422c-830a-9e223900ad64'
[09:00:16] [Thread-41/WARN]: Tried to contact Mojang page for UUID lookup but failed.
[09:00:16] [Craft Scheduler Thread - 28/WARN]: Exception in thread "Craft Scheduler Thread - 28"
[09:00:16] [Craft Scheduler Thread - 28/WARN]: org.apache.commons.lang.UnhandledException: Plugin Autorank v3.3-Dev-305 generated an exception while executing task 66
	at org.bukkit.craftbukkit.v1_7_R4.scheduler.CraftAsyncTask.run(CraftAsyncTask.java:56)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
	at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.IllegalArgumentException: Null or empty name passed! Name must not be empty
	at ru.tehkode.permissions.PermissionManager.getUser(PermissionManager.java:162)
	at net.milkbowl.vault.permission.plugins.Permission_PermissionsEx.playerHas(Permission_PermissionsEx.java:208)
	at net.milkbowl.vault.permission.Permission.playerHas(Permission.java:145)
	at me.armar.plugins.autorank.leaderboard.Leaderboard.getSortedPlaytimes(Leaderboard.java:106)
	at me.armar.plugins.autorank.leaderboard.Leaderboard.updateLeaderboard(Leaderboard.java:185)
	at me.armar.plugins.autorank.leaderboard.Leaderboard$2.run(Leaderboard.java:84)
	at org.bukkit.craftbukkit.v1_7_R4.scheduler.CraftTask.run(CraftTask.java:71)
	at org.bukkit.craftbukkit.v1_7_R4.scheduler.CraftAsyncTask.run(CraftAsyncTask.java:53)
	... 3 more