I don't understand what stacktrace is expected for this particular error. This error says that the FieldTypeHashGenerator can't generate a hash from an object. It is meant to serialize the values returned by the fromHash() method from the FieldType API, that is not supposed to return objects. The Exception described in this issue isn't bubbling up from the FieldType. It is thrown in a switch/case's default statement, so there is no "previous" exception.
I might be missing something, and maybe it tries to serialize an exception, but I don't really understand how that would happen: if an exception is thrown, it will bubble up until it is caught, and will interrupt this switch case. As mentioned, REST exceptions include the whole exception stack recursively since v1.5.