Hoola. He entrado hace un momento a una Age (creo que la 57 según el Log) a devastar medio desierto y me ha reventado el juego según he cruzado el portal. Al reloguear, más de lo mismo.
He podido recuperar el fragmento del log del cliente en el que cuenta la causa del error, para quien lo entienda y por si hubiese una solución distinta al borrado de .dat:
Spoiler
2014-05-02 09:53:49 [Información] [STDERR] java.lang.IllegalArgumentException: Failed to register dimension for id 9, One is already registered
2014-05-02 09:53:49 [Información] [STDERR] at net.minecraftforge.common.DimensionManager.registerDimension(DimensionManager.java:127)
2014-05-02 09:53:49 [Información] [STDERR] at com.xcompwiz.mystcraft.network.MPacketDimensions.registerDimensions(MPacketDimensions.java:41)
2014-05-02 09:53:49 [Información] [STDERR] at com.xcompwiz.mystcraft.network.MPacketDimensions.handle(MPacketDimensions.java:30)
2014-05-02 09:53:49 [Información] [STDERR] at com.xcompwiz.mystcraft.network.MystcraftPacketHandler.onPacketData(MystcraftPacketHandler.java:45)
2014-05-02 09:53:49 [Información] [STDERR] at cpw.mods.fml.common.network.NetworkRegistry.handlePacket(NetworkRegistry.java:255)
2014-05-02 09:53:49 [Información] [STDERR] at cpw.mods.fml.common.network.NetworkRegistry.handleCustomPacket(NetworkRegistry.java:245)
2014-05-02 09:53:49 [Información] [STDERR] at cpw.mods.fml.common.network.FMLNetworkHandler.handlePacket250Packet(FMLNetworkHandler.java:85)
2014-05-02 09:53:49 [Información] [STDERR] at net.minecraft.client.multiplayer.NetClientHandler.func_72501_a(NetClientHandler.java:1651)
2014-05-02 09:53:49 [Información] [STDERR] at net.minecraft.network.packet.Packet250CustomPayload.func_73279_a(SourceFile:61)
2014-05-02 09:53:49 [Información] [STDERR] at net.minecraft.network.TcpConnection.func_74428_b(TcpConnection.java:462)
2014-05-02 09:53:49 [Información] [STDERR] at net.minecraft.client.multiplayer.NetClientHandler.func_72551_d(NetClientHandler.java:281)
2014-05-02 09:53:49 [Información] [STDERR] at net.minecraft.client.multiplayer.GuiConnecting.func_73876_c(GuiConnecting.java:51)
2014-05-02 09:53:49 [Información] [STDERR] at net.minecraft.client.Minecraft.func_71407_l(Minecraft.java:1580)
2014-05-02 09:53:49 [Información] [STDERR] at net.minecraft.client.Minecraft.func_71411_J(Minecraft.java:910)
2014-05-02 09:53:49 [Información] [STDERR] at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:838)
2014-05-02 09:53:49 [Información] [STDERR] at net.minecraft.client.main.Main.main(SourceFile:101)
2014-05-02 09:53:49 [Información] [STDERR] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
2014-05-02 09:53:49 [Información] [STDERR] at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
2014-05-02 09:53:49 [Información] [STDERR] at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
2014-05-02 09:53:49 [Información] [STDERR] at java.lang.reflect.Method.invoke(Unknown Source)
2014-05-02 09:53:49 [Información] [STDERR] at net.minecraft.launchwrapper.Launch.launch(Launch.java:131)
2014-05-02 09:53:49 [Información] [STDERR] at net.minecraft.launchwrapper.Launch.main(Launch.java:27)
2014-05-02 09:53:52 [Información] [STDERR] net.minecraft.util.ReportedException: Ticking screen
2014-05-02 09:53:52 [Información] [STDERR] at net.minecraft.client.Minecraft.func_71407_l(Minecraft.java:1587)
2014-05-02 09:53:52 [Información] [STDERR] at net.minecraft.client.Minecraft.func_71411_J(Minecraft.java:910)
2014-05-02 09:53:52 [Información] [STDERR] at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:838)
2014-05-02 09:53:52 [Información] [STDERR] at net.minecraft.client.main.Main.main(SourceFile:101)
2014-05-02 09:53:52 [Información] [STDERR] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
2014-05-02 09:53:52 [Información] [STDERR] at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
2014-05-02 09:53:52 [Información] [STDERR] at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
2014-05-02 09:53:52 [Información] [STDERR] at java.lang.reflect.Method.invoke(Unknown Source)
2014-05-02 09:53:52 [Información] [STDERR] at net.minecraft.launchwrapper.Launch.launch(Launch.java:131)
2014-05-02 09:53:52 [Información] [STDERR] at net.minecraft.launchwrapper.Launch.main(Launch.java:27)
2014-05-02 09:53:52 [Información] [STDERR] Caused by: java.lang.IllegalArgumentException: Could not get provider type for dimension 57, does not exist
2014-05-02 09:53:52 [Información] [STDERR] at net.minecraftforge.common.DimensionManager.getProviderType(DimensionManager.java:157)
2014-05-02 09:53:52 [Información] [STDERR] at net.minecraftforge.common.DimensionManager.createProviderFor(DimensionManager.java:311)
2014-05-02 09:53:52 [Información] [STDERR] at net.minecraft.world.WorldProvider.func_76570_a(WorldProvider.java:209)
2014-05-02 09:53:52 [Información] [STDERR] at net.minecraft.client.multiplayer.WorldClient.<init>(WorldClient.java:62)
2014-05-02 09:53:52 [Información] [STDERR] at net.minecraft.client.multiplayer.NetClientHandler.func_72455_a(NetClientHandler.java:349)
2014-05-02 09:53:52 [Información] [STDERR] at net.minecraft.network.packet.Packet1Login.func_73279_a(Packet1Login.java:119)
2014-05-02 09:53:52 [Información] [STDERR] at net.minecraft.network.TcpConnection.func_74428_b(TcpConnection.java:462)
2014-05-02 09:53:52 [Información] [STDERR] at net.minecraft.client.multiplayer.NetClientHandler.func_72551_d(NetClientHandler.java:281)
2014-05-02 09:53:52 [Información] [STDERR] at net.minecraft.client.multiplayer.GuiConnecting.func_73876_c(GuiConnecting.java:51)
2014-05-02 09:53:52 [Información] [STDERR] at net.minecraft.client.Minecraft.func_71407_l(Minecraft.java:1580)
2014-05-02 09:53:52 [Información] [STDERR] ... 9 more