Server crashes! How to properly install plugin?

Server Discussion
1

I have a little server to play with friends, I set up Forge for MC 1.8 and Sponge, worked fine, now I’m trying to add plugins, but the server stops working if I add any plugin.
Shouldn’t the plugins be put in the mods folder? If I do so, the server crashes and doesn’t start.
What do I do?
Forge and Sponge version: 1563
Here is the crash log:

---- Minecraft Crash Report ----

WARNING: coremods are present:
SpongeCoremod (spongeforge-1.8-1563-2.1-DEV-814.jar)
Contact their authors BEFORE contacting forge

// I’m sorry, Dave.

Time: 5/8/16 1:55 PM
Description: Exception in server tick loop

java.lang.ClassCastException: java.util.ArrayList cannot be cast to java.lang.String
at org.spongepowered.mod.plugin.SpongeModPluginContainer.getSortingRules(SpongeModPluginContainer.java:221)
at net.minecraftforge.fml.common.Loader.sortModList(Loader.java:298)
at net.minecraftforge.fml.common.Loader.loadMods(Loader.java:487)
at net.minecraftforge.fml.server.FMLServerHandler.beginServerLoading(FMLServerHandler.java:87)
at net.minecraftforge.fml.common.FMLCommonHandler.onServerStart(FMLCommonHandler.java:355)
at net.minecraft.server.dedicated.DedicatedServer.func_71197_b(DedicatedServer.java:117)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:438)
at java.lang.Thread.run(Thread.java:745)

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

– System Details –
Details:
Minecraft Version: 1.8
Operating System: Linux (amd64) version 2.6.32-573.22.1.el6.x86_64
Java Version: 1.8.0_91, Oracle Corporation
Java VM Version: OpenJDK 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 345580976 bytes (329 MB) / 518979584 bytes (494 MB) up to 4151836672 bytes (3959 MB)
JVM Flags: 4 total; -Xmx4096M -Xms512M -XX:MaxMetaspaceSize=256M -XX:+UseConcMarkSweepGC
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
FML: MCP v9.10 FML v8.0.99.99 Minecraft Forge 11.14.4.1563 7 mods loaded, 7 mods active
States: ‘U’ = Unloaded ‘L’ = Loaded ‘C’ = Constructed ‘H’ = Pre-initialized ‘I’ = Initialized ‘J’ = Post-initialized ‘A’ = Available ‘D’ = Disabled ‘E’ = Errored
U mcp{9.05} [Minecraft Coder Pack] (minecraft.jar)
U FML{8.0.99.99} [Forge Mod Loader] (custom_server.jar)
U Forge{11.14.4.1563} [Minecraft Forge] (custom_server.jar)
U Sponge{1.8-1563-2.1-DEV-814} [SpongeForge] (minecraft.jar)
U bedrock{1.0} [bedrock] (Bedrock-1.0-beta.jar)
U io.github.hsyyid.essentialcmds{8.1.2-0423f51} [EssentialCmds] (EssentialCmds-8.1.2.jar)
U worldedit{6.1} [WorldEdit] (worldedit-forge-mc1.8-6.1.jar)
Loaded coremods (and transformers):
SpongeCoremod (spongeforge-1.8-1563-2.1-DEV-814.jar)

Profiler Position: N/A (disabled)
Is Modded: Definitely; Server brand changed to 'fml,forge,SpongeForge'
Type: Dedicated Server (map_server.txt)
2

You’re running an extremely outdated build of SpongeForge. You can download the latest build here.

3

Your SpongeForge is well over a couple hundred builds out of date (latest is build 1317). To continue using newer plugins for the desired API version, you should try to match the API version of the implementation with the Plugins targeted API version.

4

If you need to stick to Forge 1.8 (not 1.8.9), then use 3.1.0-BETA-1045 as it’s the latest SpongeForge build for 1.8. But note that it uses API version 3.1.0… We’re at 4.x now and plugins written for 4.x aren’t compatible to 3.x.

I strongly advise you to update Forge to 1.8.9 and use the latest SpongeForge build.