Every few months I run into this because I forget that version 5 of the Jwt token library introduces a breaking change that makes it incompatible with OWIN/Katana v3.x self hosted apps. So, by having forgotten and updating my nuget packages, my app starts failling again.
Hopefully by writing a quick post I will remember this issue in the future. The fix is simple. Just downgrade your System.IdentityModel.Tokens.Jwt package back to version 4.0.4.x and it will start working again.