lets say ive got a pc and a laptop.
i start a LAN world on the pc with all packs and want to join with my laptop which has also all packs.
but i cant because the itemspawner and weaponblock are 4073 and 4074
but on my laptop they are 4093 and 4094...
also i copied the whole .minecraft to my laptop but the ids are still 20 higher... help??

I just tried to join a local world with flans 2.3 with all of the flans packs and just got a read timeout error.

Latest compiled version, error message when trying to join is now "Internal server error".

The output-server.log of the client gives this error:

2013-05-31 13:16:15 [SERVER] [INFO] Disconnecting groverfield [/192.168.0.22:52441]: Internal server error
2013-05-31 13:16:15 [SERVER] [WARNING] Failed to handle packet for groverfield [/192.168.0.22:52441]: java.lang.NullPointerException
java.lang.NullPointerException
at cpw.mods.fml.common.modloader.ModLoaderNetworkHandler.acceptVersion(ModLoaderNetworkHandler.java:49)
at cpw.mods.fml.common.network.ModListResponsePacket.execute(ModListResponsePacket.java:115)
at cpw.mods.fml.common.network.FMLNetworkHandler.handleFMLPacket(FMLNetworkHandler.java:115)
at cpw.mods.fml.common.network.FMLNetworkHandler.handlePacket250Packet(FMLNetworkHandler.java:80)
at net.minecraft.network.NetLoginHandler.func_72501_a(NetLoginHandler.java:326)
at net.minecraft.network.packet.Packet250CustomPayload.func_73279_a(SourceFile:59)
at net.minecraft.network.TcpConnection.func_74428_b(TcpConnection.java:461)
at net.minecraft.network.NetLoginHandler.func_72532_c(NetLoginHandler.java:80)
at net.minecraft.server.ServerListenThread.func_71766_a(ServerListenThread.java:55)
at net.minecraft.server.integrated.IntegratedServerListenThread.func_71747_b(IntegratedServerListenThread.java:106)
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:675)
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:571)
at net.minecraft.server.integrated.IntegratedServer.func_71217_p(IntegratedServer.java:171)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:469)
at net.minecraft.server.ThreadMinecraftServer.run(SourceFile:573)

Single player LAN works fine for me, might be an issue with different IDs on client/server. Shouldn't be a problem with 1.7

It has always worked when run through eclipse.
I'll test again tomorrow if that's not what you're doing.

No I was testing in Eclipse, looks like a forge bug from the error message. However an updated error message would be good

Just tried this again with two different launchers and it worked. It would seem that I can't connect to the LAN server when using MagicLauncher but the user using the default launcher can connect.