I do have other mods installed that may contribute to the problem but the server runs fine when I remove Serene Seasons. Here is the log.

---- Minecraft Crash Report ----

WARNING: coremods are present:
SSLoadingPlugin (SereneSeasons-1.12.2-1.2.1-universal.jar)
IELoadingPlugin (ImmersiveEngineering-0.12-82-core.jar)
LoadingPlugin (ChunkAnimator-MC1.12-1.2.jar)
Contact their authors BEFORE contacting forge

// Why did you do that?

Time: 5/18/18 9:25 PM
Description: Exception in server tick loop

net.minecraftforge.fml.common.LoaderExceptionModCrash: Caught exception from Serene Seasons (sereneseasons)
Caused by: java.lang.NoClassDefFoundError: net/minecraft/client/util/ITooltipFlag
at java.lang.Class.getDeclaredConstructors0(Native Method)
at java.lang.Class.privateGetDeclaredConstructors(Unknown Source)
at java.lang.Class.getConstructor0(Unknown Source)
at java.lang.Class.getConstructor(Unknown Source)
at net.minecraftforge.fml.common.eventhandler.EventBus.register(EventBus.java:127)
at net.minecraftforge.fml.common.eventhandler.EventBus.register(EventBus.java:111)
at sereneseasons.init.ModHandlers.init(ModHandlers.java:39)
at sereneseasons.core.SereneSeasons.preInit(SereneSeasons.java:46)
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.GeneratedMethodAccessor9.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)
... 43 more
Caused by: java.lang.NullPointerException

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_171, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 903808360 bytes (861 MB) / 1118306304 bytes (1066 MB) up to 3804758016 bytes (3628 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 27 mods loaded, 27 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-1.12.2-14.23.3.2655-universal.jar       | e3c3d50c7c986df74c645c0ac54639741c90a557 |
| UCH   | forge                | 14.23.3.2655   | forge-1.12.2-14.23.3.2655-universal.jar       | e3c3d50c7c986df74c645c0ac54639741c90a557 |
| UCH   | biomesoplenty        | 7.0.1.2384     | BiomesOPlenty-1.12.2-7.0.1.2384-universal.jar | None                                     |
| UCH   | craftstudioapi       | 1.0.0          | CraftStudio-1.0.0.93-mc1.12-alpha.jar         | None                                     |
| UCH   | animania             | 1.4.7          | animania-1.12.2-1.4.8.jar                     | None                                     |
| UCH   | astikoor             | 1.0.0          | astikoor-1.0.0.jar                            | None                                     |
| UCH   | chunkpregenerator    | 1.8            | Chunk Pregenerator V1.12-1.8.jar              | None                                     |
| UCH   | extendedrenderer     | v1.0           | coroutil-1.12.1-1.2.10.jar                    | None                                     |
| UCH   | coroutil             | 1.12.1-1.2.10  | coroutil-1.12.1-1.2.10.jar                    | None                                     |
| UCH   | configmod            | v1.0           | coroutil-1.12.1-1.2.10.jar                    | None                                     |
| UCH   | craftablehorsearmour | 1.3            | CraftableHorseArmour-1.3.0-1.12.jar           | None                                     |
| UCH   | dynamictrees         | 1.12.2-0.7.6   | DynamicTrees-1.12.2-0.7.6.jar                 | None                                     |
| UCH   | dynamictreesbop      | 1.0.0          | dynamictreesbop-1.0.0.jar                     | None                                     |
| UCH   | fastleafdecay        | v14            | FastLeafDecay-v14.jar                         | None                                     |
| UCH   | ichunutil            | 7.1.4          | iChunUtil-1.12.2-7.1.4.jar                    | None                                     |
| UCH   | immersiveengineering | 0.12-82        | ImmersiveEngineering-0.12-82.jar              | 4cb49fcde3b43048c9889e0a3d083225da926334 |
| UCH   | immersivepetroleum   | 1.1.9          | immersivepetroleum-1.12.2-1.1.9.jar           | None                                     |
| UCH   | radixcore            | 1.12.x-2.2.1   | RadixCore-1.12.x-2.2.1-universal.jar          | None                                     |
| UCH   | mca                  | 1.12.2-5.3.1   | MCA-1.12.x-5.3.1-universal.jar                | None                                     |
| UCE   | sereneseasons        | 1.2.1          | SereneSeasons-1.12.2-1.2.1-universal.jar      | None                                     |
| UCH   | vc                   | 5.9.13         | ViesCraft-1.12.2-5.9.13.jar                   | None                                     |
| UCH   | wanionlib            | 1.12.2-1.5     | WanionLib-1.12.2-1.5.jar                      | None                                     |
| UCH   | wawla                | 2.5.253        | Wawla-1.12.2-2.5.253.jar                      | d476d1b22b218a10d845928d1665d45fce301b27 |
| UCH   | zombieawareness      | 1.12.1-1.11.12 | zombieawareness-1.12.1-1.11.12.jar            | None                                     |
| UCH   | unidict              | 1.12.2-2.5c    | UniDict-1.12.2-2.5c.jar                       | None                                     |

Loaded coremods (and transformers): 

SSLoadingPlugin (SereneSeasons-1.12.2-1.2.1-universal.jar)
sereneseasons.asm.transformer.EntityRendererTransformer
sereneseasons.