Help please!

20:19:10 [SEVERE] Error occurred while enabling GroupManager v1.7 (Phoenix) (Is
it up to date?): java.lang.String cannot be cast to java.util.Map
java.lang.ClassCastException: java.lang.String cannot be cast to java.util.Map
at org.anjocaido.groupmanager.dataholder.WorldDataHolder.loadGroups(Worl
dDataHolder.java:613)
at org.anjocaido.groupmanager.dataholder.WorldDataHolder.load(WorldDataH
older.java:571)
at org.anjocaido.groupmanager.dataholder.worlds.WorldsHolder.loadWorld(W
orldsHolder.java:460)
at org.anjocaido.groupmanager.dataholder.worlds.WorldsHolder.mirrorSetUp
(WorldsHolder.java:121)
at org.anjocaido.groupmanager.dataholder.worlds.WorldsHolder.initialLoad
(WorldsHolder.java:68)
at org.anjocaido.groupmanager.dataholder.worlds.WorldsHolder.<init>(Worl
dsHolder.java:58)
at org.anjocaido.groupmanager.GroupManager.onEnable(GroupManager.java:13
1)
at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:188)
at org.bukkit.plugin.java.JavaPluginLoader.enablePlugin(JavaPluginLoader
.java:968)
at org.bukkit.plugin.SimplePluginManager.enablePlugin(SimplePluginManage
r.java:280)
at org.bukkit.craftbukkit.CraftServer.loadPlugin(CraftServer.java:186)
at org.bukkit.craftbukkit.CraftServer.enablePlugins(CraftServer.java:169
)
at net.minecraft.server.MinecraftServer.t(MinecraftServer.java:348)
at net.minecraft.server.MinecraftServer.a(MinecraftServer.java:335)
at net.minecraft.server.MinecraftServer.init(MinecraftServer.java:165)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:399)
at net.minecraft.server.ThreadServerApplication.run(SourceFile:457)
20:19:10 [INFO] Essentials: Using GroupManager based permissions.
20:19:10 [INFO] CButD: CraftBukkit is up to Date
20:19:10 [INFO] Initializing c3p0-0.9.1.2 [built 21-May-2007 15:04:56; debug? tr
ue; trace: 10]

What does this mean? The error part, I wrote all permissions right and it's not working because of this