[20:28:39 WARN]: [Quests] Task #93 for Quests v2.6.0-b98 generated an exception
java.lang.NoClassDefFoundError: net/minecraft/server/v1_8_R3/Packet
at me.blackvein.quests.NpcEffectThread.showEffect_1_9_R1(NpcEffectThread
.java:134) ~[?:?]
at me.blackvein.quests.NpcEffectThread.showEffect(NpcEffectThread.java:7
9) ~[?:?]
at me.blackvein.quests.NpcEffectThread.run(NpcEffectThread.java:45) ~[?:
?]
at org.bukkit.craftbukkit.v1_9_R1.scheduler.CraftTask.run(CraftTask.java
:53) ~[custom.jar:git-Bukkit-712213d]
at org.bukkit.craftbukkit.v1_9_R1.scheduler.CraftScheduler.mainThreadHea
rtbeat(CraftScheduler.java:349) [custom.jar:git-Bukkit-712213d]
at net.minecraft.server.v1_9_R1.MinecraftServer.D(MinecraftServer.java:6
86) [custom.jar:git-Bukkit-712213d]
at net.minecraft.server.v1_9_R1.DedicatedServer.D(DedicatedServer.java:3
61) [custom.jar:git-Bukkit-712213d]
at net.minecraft.server.v1_9_R1.MinecraftServer.C(MinecraftServer.java:6
35) [custom.jar:git-Bukkit-712213d]
at net.minecraft.server.v1_9_R1.MinecraftServer.run(MinecraftServer.java
:539) [custom.jar:git-Bukkit-712213d]
at java.lang.Thread.run(Unknown Source) [?:1.8.0_77]
Caused by: java.lang.ClassNotFoundException: net.minecraft.server.v1_8_R3.Packet

    at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.

java:67) ~[custom.jar:git-Bukkit-712213d]
at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.
java:62) ~[custom.jar:git-Bukkit-712213d]
at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_77]
at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_77]
... 10 more

no idea what this is related to... happens upon me logging onto the server
I am using the craftbukkit.jar created by the latest BuildTools as of 3/30/16
also using the latest Citizens2 and Denizens jarfile

It's related to the NPC Particles that indicate it has a quest. Looks like this was fixed:
PikaMug/Quests@1329fe4

Give it a try, here's the version which includes that code, so if it fixes your problem I suggest clicking the Closed button to close this issue.

Alternatively, if this DOESN'T fix it for you, I had this issue when 1.8 came out, and just going into the config to disable the particles will at least prevent this from happening.

Fixed in build mentioned by @Formula350