After viewing a backpack and pressing escape again return to the original player inventory view, this error appears in the server console:

Failed to run a 0.10 timer in 'InventoryViewer v4.1.0' (ArgumentException: An item with the same key has already been added. Key: 76561############-76561############)
  at System.Collections.Generic.Dictionary`2[TKey,TValue].TryInsert (TKey key, TValue value, System.Collections.Generic.InsertionBehavior behavior) [0x0015a] in <e3aa8db986d7457ca73f10110f1283f5>:0
  at System.Collections.Generic.Dictionary`2[TKey,TValue].Add (TKey key, TValue value) [0x00000] in <e3aa8db986d7457ca73f10110f1283f5>:0
  at Oxide.Plugins.InventoryViewer.StartLooting (BasePlayer player, BasePlayer targetplayer, LootableCorpse corpse) [0x000cb] in <7ba937d815d34972ae8c57920a493bb6>:0
  at Oxide.Plugins.InventoryViewer+<>c__DisplayClass13_0.<ViewInventory>b__1 () [0x00000] in <7ba937d815d34972ae8c57920a493bb6>:0
  at Oxide.Core.Libraries.Timer+TimerInstance.FireCallback () [0x00018] in <beb2b64691c64e2b95b99491bd85442c>:0

same error here..

This is fixed in 4.1.2