This can occur when a method is declared public in a controller, but without REST attributes. Changing the method to protected may address the issue.
I have seen this kind of error before and usually the error message points to the culprit:
EBisAPI.Controllers._class.HandleError
I guess HandleError is a public method in your base class, right? Change it to protected and try again.
This is of course only one possible solution. If the method which is mentioned in the error message is part of an interface implementation, it doesn’t work and you need to look at one of the other solutions.