After updating to 3.21 I've noticed that this happens. If u can't reproduce, I can give logs & info.

Yes, but the client should have the fml-client-latest.log. Perhaps you should describe a bit what you're doing exactly? It sounds like you're using a server? What about the log on the client side? How are you starting MC? What version of Java?

Without logs I'm going to need more detailed information.

BTW. You have mcjtylib1.1.0 right? But you hopefully didn't forget to remove an older version of mcjtylib if you have it? Also server and client are both using same version?

I've updated from 3.11, when no mcjtylib is needed and of course have removed previous version from server and client. I've checked that in singleplayer, it happens. And one and only log present, latest log, have
[00:36:47] [Client thread/INFO]: net.minecraft.client.gui.inventory.GuiContainerCreative
[00:36:47] [Client thread/INFO]: net.minecraft.client.gui.inventory.GuiContainerCreative$ContainerCreative
[00:36:59] [Client thread/INFO]: mcjty.rftools.blocks.crafter.GuiCrafter
[00:36:59] [Client thread/INFO]: mcjty.rftools.blocks.crafter.CrafterContainer
[00:37:01] [Server thread/ERROR]: SimpleChannelHandlerWrapper exception
java.lang.NullPointerException
at mcjty.network.PacketSetGuiStyle.onMessage(PacketSetGuiStyle.java:40) ~[PacketSetGuiStyle.class:?]
at mcjty.network.PacketSetGuiStyle.onMessage(PacketSetGuiStyle.java:14) ~[PacketSetGuiStyle.class:?]
at cpw.mods.fml.common.network.simpleimpl.SimpleChannelHandlerWrapper.channelRead0(SimpleChannelHandlerWrapper.java:37) ~[SimpleChannelHandlerWrapper.class:?]
at cpw.mods.fml.common.network.simpleimpl.SimpleChannelHandlerWrapper.channelRead0(SimpleChannelHandlerWrapper.java:17) ~[SimpleChannelHandlerWrapper.class:?]
at io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:98) ~[SimpleChannelInboundHandler.class:?]
at io.netty.channel.DefaultChannelHandlerContext.invokeChannelRead(DefaultChannelHandlerContext.java:337) [DefaultChannelHandlerContext.class:?]
at io.netty.channel.DefaultChannelHandlerContext.fireChannelRead(DefaultChannelHandlerContext.java:323) [DefaultChannelHandlerContext.class:?]
at io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:101) [SimpleChannelInboundHandler.class:?]
at io.netty.channel.DefaultChannelHandlerContext.invokeChannelRead(DefaultChannelHandlerContext.java:337) [DefaultChannelHandlerContext.class:?]
...

And, still REALLY no fml-client-latest.log. I can share the screen via skype :)
MC starting with official client launcher v.1.6.19. Java:
java version "1.8.0_25"
Java(TM) SE Runtime Environment (build 1.8.0_25-b18)
Java HotSpot(TM) 64-Bit Server VM (build 25.25-b02, mixed mode)

Server also have mcjtylib-1.1.0.jar and prev version deleted. Anything else?

Oh Java 1.8.0_25. Please upgrade that to a more recent Java8. There were problems with that version of Java. Let me know if that helps.

java version "1.8.0_51"
Java(TM) SE Runtime Environment (build 1.8.0_51-b16)
Java HotSpot(TM) 64-Bit Server VM (build 25.51-b03, mixed mode)

It is not helps, even on singleplayer.

Well then I have no clue. Others have not reported any errors with latest rftools and mcjtylib. Without a more detailed log I'm afraid there isn't much I can do :-(

Works fine for me so I would appreciate a full client log.

Client log contain no errors, but server console... http://dl.netlocal.ru/mc/.logs/rft_server-log.txt.zip

I really need the full log. Including things that are no errors. It is all relevant to find out what is wrong here.

Ok, here is it: http://dl.netlocal.ru/mc/.logs/rft_client-log.zip

Still not the right one. The real log is MUCH larger. I'm talking about fml-client-latest.log

That's strange. Seems to be my client is not writing it. It's just latest.log in .minecraft\logs and archive of previous logs.

hmm. Without that information I'm a bit blind I'm afraid. Can you try again and see if a log is generated then? Thanks

Still the same. Server have fml-server-latest.log, but not client (I mean fml-client-latest.log).

Ok, seems I can reproduce this after all. I'll work on a fix. Sorry for bothering you about logs :-)

Not a problem, I glad to help. Btw, what problem it was? Some mod is conflicting with yours on GUI?

Will be fixed in next release (probably 3.30)