HappyHour was compiled successfully in 2126ms
Failed to call hook 'OnServerInitialized' on plugin 'HappyHour v2.0.1' (KeyNotFoundException: The given key was not present in the dictionary.)
  at System.Collections.Generic.Dictionary`2[TKey,TValue].get_Item (TKey key) [0x0001e] in <e1a80661d61443feb3dbdaac88eeb776>:0

  at Oxide.Plugins.HappyHour.insidetimer () [0x0007f] in <d91a8d557fbc48b09529ba5bafd6e3b8>:0

  at Oxide.Plugins.HappyHour.OnServerInitialized () [0x00000] in <d91a8d557fbc48b09529ba5bafd6e3b8>:0

  at Oxide.Plugins.HappyHour.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00302] in <d91a8d557fbc48b09529ba5bafd6e3b8>:0

  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <9affce1cd15c4ec183941adef8db1722>:0

  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <4452f821def6406d834e4149849fe7ea>:0

  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <4452f821def6406d834e4149849fe7ea>:0

Loaded plugin Happy Hour Plugin v2.0.1 by BuzZ

I know this post is super old, but as of July 2019's wipe, Happy Hour stopped functioning altogether. Attempts at removing Happy Hour and all related files, and then reinstalling it, give me the exact same errors as the OP. I think this plugin may need to be updated to be compatible with the July 2019 patch. Tried a server restart before attempting to load Happy Hour with the same effect.

I know this topic is old,
but I made an update