Hi
If the client config is set to the default dimension 2 and in my case the server side is set to 200 because I have dimension conflicting with a other mod, the client side crashes.
In order to fix this i need to edit the player data file so he/she spawn in overworld, after setting the player config file to the same dimension as the server.

I do get to types of error:

ava.lang.IllegalArgumentException: Could not get provider type for dimension 200, does not exist
at net.minecraftforge.common.DimensionManager.getProviderType(DimensionManager.java:148)
at net.minecraftforge.common.DimensionManager.createProviderFor(DimensionManager.java:302)
at net.minecraft.world.WorldProvider.func_76570_a(WorldProvider.java:160)
at net.minecraft.client.multiplayer.WorldClient.(WorldClient.java:54)
at net.minecraft.client.network.NetHandlerPlayClient.func_147280_a(NetHandlerPlayClient.java:865)
at net.minecraft.network.play.server.S07PacketRespawn.func_148833_a(SourceFile:32)
at net.minecraft.network.play.server.S07PacketRespawn.func_148833_a(SourceFile:13)
at net.minecraft.network.NetworkManager.func_74428_b(NetworkManager.java:212)
at net.minecraft.client.multiplayer.PlayerControllerMP.func_78765_e(PlayerControllerMP.java:273)
at net.minecraft.client.Minecraft.func_71407_l(Minecraft.java:1591)
at net.minecraft.client.Minecraft.func_71411_J(Minecraft.java:962)
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:887)
at net.minecraft.client.main.Main.main(SourceFile:148)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:483)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:135)
at net.minecraft.launchwrapper.Launch.main(Launch.java:28)

A detailed walkthrough of the error, its code path and all known details is as follows:

-- Head --
Stacktrace:
at net.minecraftforge.common.DimensionManager.getProviderType(DimensionManager.java:148)
at net.minecraftforge.common.DimensionManager.createProviderFor(DimensionManager.java:302)
at net.minecraft.world.WorldProvider.func_76570_a(WorldProvider.java:160)
at net.minecraft.client.multiplayer.WorldClient.(WorldClient.java:54)
at net.minecraft.client.network.NetHandlerPlayClient.func_147280_a(NetHandlerPlayClient.java:865)
at net.minecraft.network.play.server.S07PacketRespawn.func_148833_a(SourceFile:32)
at net.minecraft.network.play.server.S07PacketRespawn.func_148833_a(SourceFile:13)
at net.minecraft.network.NetworkManager.func_74428_b(NetworkManager.java:212)
at net.minecraft.client.multiplayer.PlayerControllerMP.func_78765_e(PlayerControllerMP.java:273)

On second attempt if the config are NOT changed

java.lang.IllegalArgumentException: Could not get provider type for dimension -56, does not exist
at net.minecraftforge.common.DimensionManager.getProviderType(DimensionManager.java:148)
at net.minecraftforge.common.DimensionManager.createProviderFor(DimensionManager.java:302)
at net.minecraft.world.WorldProvider.func_76570_a(WorldProvider.java:160)
at net.minecraft.client.multiplayer.WorldClient.(WorldClient.java:54)
at net.minecraft.client.network.NetHandlerPlayClient.func_147282_a(NetHandlerPlayClient.java:237)
at net.minecraft.network.play.server.S01PacketJoinGame.func_148833_a(SourceFile:70)
at net.minecraft.network.play.server.S01PacketJoinGame.func_148833_a(SourceFile:13)
at net.minecraft.network.NetworkManager.func_74428_b(NetworkManager.java:212)
at net.minecraft.client.multiplayer.GuiConnecting.func_73876_c(SourceFile:78)
at net.minecraft.client.Minecraft.func_71407_l(Minecraft.java:1650)
at net.minecraft.client.Minecraft.func_71411_J(Minecraft.java:962)
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:887)
at net.minecraft.client.main.Main.main(SourceFile:148)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:483)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:135)
at net.minecraft.launchwrapper.Launch.main(Launch.java:28)

A detailed walkthrough of the error, its code path and all known details is as follows:

-- Head --
Stacktrace:
at net.minecraftforge.common.DimensionManager.getProviderType(DimensionManager.java:148)
at net.minecraftforge.common.DimensionManager.createProviderFor(DimensionManager.java:302)
at net.minecraft.world.WorldProvider.func_76570_a(WorldProvider.java:160)
at net.minecraft.client.multiplayer.WorldClient.(WorldClient.java:54)
at net.minecraft.client.network.NetHandlerPlayClient.func_147282_a(NetHandlerPlayClient.java:237)
at net.minecraft.network.play.server.S01PacketJoinGame.func_148833_a(SourceFile:70)
at net.minecraft.network.play.server.S01PacketJoinGame.func_148833_a(SourceFile:13)
at net.minecraft.network.NetworkManager.func_74428_b(NetworkManager.java:212)
at net.minecraft.client.multiplayer.GuiConnecting.func_73876_c(SourceFile:78)

The used dimension should be determined automatically and is only 2 because that was the next free dimension id on your server when it first started. Which mod is the conflicting dimension from?
Someone is reserving dimension ids at the wrong time - not saying it isn't me ;)

It did conflict with Mystcraft, can´t remember which version I did use the first time.
Who is reserving the ID´s at the wrong time I can´t tell, but my guess is that XCompWiz have more experiences on this area ;-)

But my biggest issue was that I needed to update the config file on the client and server side to fix this.
If you enter a CM and the config file on the client side is not set to the same dimension as the server side this happened.

Just let me know if there is any more information you need :-)

I, too, have the same issue. Server kept crashing with the default dim id. Looked like a dimensional conflict even though there was no dim 2 in use. Mystcraft uses positive dims so I set mine to -95. Server started but I crashed when using the psd on a machine. marduv's suggestion did the trick.