Quests version 2.6.5-b104
spigot version: git-Spigot-a9631d2-56813c7 (MC: 1.10.2)

I'm seeing the following error repeatedly in the console:
http://pastebin.com/SCEHQvUv

Closing due to lack of response from OP.

Im no expert - but are you using an updated version of citizens as well? Quests really demands everything be current- that means latest spigot buildtools version, latest citizens, latest Quests, latest Sentinel .jar (Sentry has been abandoned and has become Sentinels), and possible the latest Protocollib .jar

You will need to provide steps to reproduce this problem, as well as your Quests config. Without that we can't diagnose the problem.