[Sync Pipes] Save v1.0 starting
[Sync Pipes] Saved 344 pipes
[Sync Pipes] Saved 260 managers
[Sync Pipes] Save v1.0 complete (0.07s)
[Sync Pipes] Unloading All Pipes
Failed to call hook 'Unload' on plugin 'SyncPipes v0.9.21' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Plugins.SyncPipes+OverlayText.Hide (BasePlayer player, System.Single delay, System.String callerName) [0x0008b] in <783984f2879349c185ba3de3df770e1d>:0

  at Oxide.Plugins.SyncPipes+PlayerHelper.Cleanup () [0x0001d] in <783984f2879349c185ba3de3df770e1d>:0

  at Oxide.Plugins.SyncPipes.Unload () [0x00022] in <783984f2879349c185ba3de3df770e1d>:0

  at Oxide.Plugins.SyncPipes.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x000fc] in <783984f2879349c185ba3de3df770e1d>:0

  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <60c318df79ed41688ea59335e48d61ad>:0

  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <9882f28dc2204b4dba514a9ad18f5042>:0

  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <9882f28dc2204b4dba514a9ad18f5042>:0

Looks like it tried to close a pipes menu on a player and it couldn't find them. I'll add some error catching for that. 

This should be fixed in V0.9.23 (beta)
https://umod.org/plugins/SyncPipes.cs?version=0.9.23&branch=beta