Go into Manage Nuget Packages for Solution...
on the solution, choose Updates
and click Update All
(or select the owin related ones as applicable).
This fixed it for me.
Original Error:
Error: System.ArgumentException No conversion available between System.Func
2[System.Collections.Generic.IDict ionary
2[System.String,System.Object],System.Threading.Tasks.Task] and Microsoft .Owin.OwinMiddleware. Parameter name: signature
After Updating Nuget Packages:
Starting with the default port: 5000 (http://localhost:5000/) Started successfully Press Enter to exit