Found one example on the whole Internet of the following, and it was on StackOverflow, but there was no follow up whether it had been solved or not. This error happened after I implemented accept header versioning with attribute routing in a WebAPI 2 project.
Message: "An error has occurred." ExceptionMessage: "The given key was not present in the dictionary." ExceptionType: "System.Collections.Generic.KeyNotFoundException"
found here on stackoverflow:
How to get controller name when Web API versioning with routing attribues