Failed to run a 0.50 timer in 'Backpacks v3.7.0' (MissingMethodException: Default constructor not found for type Oxide.Plugins.Backpacks+Backpack)
  at System.RuntimeType.CreateInstanceMono (System.Boolean nonPublic) [0x0007b] in :0 
  at System.RuntimeType.CreateInstanceSlow (System.Boolean publicOnly, System.Boolean skipCheckThis, System.Boolean fillCache, System.Threading.StackCrawlMark& stackMark) [0x00009] in :0 
  at System.RuntimeType.CreateInstanceDefaultCtor (System.Boolean publicOnly, System.Boolean skipCheckThis, System.Boolean fillCache, System.Threading.StackCrawlMark& stackMark) [0x00027] in :0 
  at System.Activator.CreateInstance[T] () [0x00015] in :0 
  at Oxide.Core.DataFileSystem.ReadObject[T] (System.String name) [0x00009] in <9882f28dc2204b4dba514a9ad18f5042>:0 
  at Oxide.Plugins.Backpacks.LoadData[T] (T& data, System.String filename) [0x0000b] in :0 
  at Oxide.Plugins.Backpacks+BackpackManager.Load (System.UInt64 userId) [0x0000e] in :0 
  at Oxide.Plugins.Backpacks+BackpackManager.GetBackpack (System.UInt64 userId) [0x00010] in :0 
  at Oxide.Plugins.Backpacks+BackpackManager.OpenBackpack (System.UInt64 backpackOwnerId, BasePlayer looter) [0x00010] in :0 
  at Oxide.Plugins.Backpacks+c__AnonStorey2.<>m__0 () [0x00000] in :0 
  at Oxide.Core.Libraries.Timer+TimerInstance.FireCallback () [0x00018] in <9882f28dc2204b4dba514a9ad18f5042>:0 

Thank you for reporting. I was able to reproduce this by deleting my user's backpack data file and attempting to open the backpack. I have toggled the update to invisible to prevent other people downloading it while I work on a fix.

Update: Fixed in v3.7.1.