Not looking at your code yet, I think I can suggest something. When a class is first loaded, in addition to storing the class's byte codes, the JVM also has tables where it keeps track of the types of fields and the signatures of methods in each class.
The error you are seeing would suggest that the class was loaded, this signature information was stored and then you tried to add the method after that.
If you instead put your agent jar onto the command line, you can do things before the class is loaded for the first time. If you add your method before the signature info is stored away, you should be good.
If you have to connect the agent after the process is already launched, you may be able to transform the class but you may only be able to transform it without changing the set of fields, their types, or the methods, or their signatures. In other words, you may be able to change the byte codes but you have to not invalidate the previously-stored meta information.