Minecraft 1.12.2
Forge 14.23.3.2655

Installed Mods:
reborncore 3.8.0.264
techreborn 2.15.0.602
(no others mods installed)

Crash occurs only in Dedicated Server, normal game client work

---- Minecraft Crash Report ----
// Ouch. That hurt :(

Time: 22.04.18 14:27
Description: Exception in server tick loop

net.minecraftforge.fml.common.LoaderExceptionModCrash: Caught exception from Reborn Core (reborncore)
Caused by: java.lang.NoClassDefFoundError: net/minecraft/client/util/ITooltipFlag
at java.lang.Class.getDeclaredMethods0(Native Method)
at java.lang.Class.privateGetDeclaredMethods(Unknown Source)
at java.lang.Class.getDeclaredMethods(Unknown Source)
at reborncore.common.registration.RegistrationManager.handleClass(RegistrationManager.java:128)
at reborncore.common.registration.RegistrationManager.load(RegistrationManager.java:103)
at reborncore.RebornCore.preInit(RebornCore.java:99)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at net.minecraftforge.fml.common.FMLModContainer.handleModStateEvent(FMLModContainer.java:600)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.google.common.eventbus.Subscriber.invokeSubscriberMethod(Subscriber.java:91)
at com.google.common.eventbus.Subscriber$SynchronizedSubscriber.invokeSubscriberMethod(Subscriber.java:150)
at com.google.common.eventbus.Subscriber$1.run(Subscriber.java:76)
at com.google.common.util.concurrent.MoreExecutors$DirectExecutor.execute(MoreExecutors.java:399)
at com.google.common.eventbus.Subscriber.dispatchEvent(Subscriber.java:71)
at com.google.common.eventbus.Dispatcher$PerThreadQueuedDispatcher.dispatch(Dispatcher.java:116)
at com.google.common.eventbus.EventBus.post(EventBus.java:217)
at net.minecraftforge.fml.common.LoadController.sendEventToModContainer(LoadController.java:278)
at net.minecraftforge.fml.common.LoadController.propogateStateMessage(LoadController.java:256)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.google.common.eventbus.Subscriber.invokeSubscriberMethod(Subscriber.java:91)
at com.google.common.eventbus.Subscriber$SynchronizedSubscriber.invokeSubscriberMethod(Subscriber.java:150)
at com.google.common.eventbus.Subscriber$1.run(Subscriber.java:76)
at com.google.common.util.concurrent.MoreExecutors$DirectExecutor.execute(MoreExecutors.java:399)
at com.google.common.eventbus.Subscriber.dispatchEvent(Subscriber.java:71)
at com.google.common.eventbus.Dispatcher$PerThreadQueuedDispatcher.dispatch(Dispatcher.java:116)
at com.google.common.eventbus.EventBus.post(EventBus.java:217)
at net.minecraftforge.fml.common.LoadController.distributeStateMessage(LoadController.java:148)
at net.minecraftforge.fml.common.Loader.preinitializeMods(Loader.java:602)
at net.minecraftforge.fml.server.FMLServerHandler.beginServerLoading(FMLServerHandler.java:98)
at net.minecraftforge.fml.common.FMLCommonHandler.onServerStart(FMLCommonHandler.java:332)
at net.minecraft.server.dedicated.DedicatedServer.func_71197_b(DedicatedServer.java:125)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:484)
at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.ClassNotFoundException: net.minecraft.client.util.ITooltipFlag
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:191)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
... 42 more
Caused by: java.lang.NullPointerException
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:182)
... 44 more

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

-- System Details --
Details:
Minecraft Version: 1.12.2
Operating System: Windows 10 (amd64) version 10.0
Java Version: 1.8.0_151, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 1114366984 bytes (1062 MB) / 1258815488 bytes (1200 MB) up to 7387742208 bytes (7045 MB)
JVM Flags: 0 total;
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
FML: MCP 9.42 Powered by Forge 14.23.3.2655 6 mods loaded, 6 mods active
States: 'U' = Unloaded 'L' = Loaded 'C' = Constructed 'H' = Pre-initialized 'I' = Initialized 'J' = Post-initialized 'A' = Available 'D' = Disabled 'E' = Errored

| State | ID         | Version      | Source                    | Signature                                |
|:----- |:---------- |:------------ |:------------------------- |:---------------------------------------- |
| UCH   | minecraft  | 1.12.2       | minecraft.jar             | None                                     |
| UCH   | mcp        | 9.42         | minecraft.jar             | None                                     |
| UCH   | FML        | 8.0.99.99    | forge.jar                 | e3c3d50c7c986df74c645c0ac54639741c90a557 |
| UCH   | forge      | 14.23.3.2655 | forge.jar                 | e3c3d50c7c986df74c645c0ac54639741c90a557 |
| UCE   | reborncore | 3.8.0.264    | RebornCore-3.8.0.264.jar  | 8727a3141c8ec7f173b87aa78b9b9807867c4e6b |
| UCE   | techreborn | 2.15.0.602   | TechReborn-2.15.0.602.jar | 8727a3141c8ec7f173b87aa78b9b9807867c4e6b |

Loaded coremods (and transformers): 
RebornCore: 
	Plugin Engine: 0
	RebornCore Version: 3.8.0.264
	Runtime Debofucsation 1
Profiler Position: N/A (disabled)
Is Modded: Definitely; Server brand changed to 'fml,forge'
Type: Dedicated Server (map_server.txt)

This was fixed 15 mins ago, TechReborn/TechReborn@08a6d30 An updated version of TechReborn should out as soon as its approved by curse.