This is just my guess since I don't know your code but it might be that you are returning just the manager ID from your participant resolver. When you have the manager ID you should use the provider accessor to find the manager user object and return it.
User oldmanageruser = ParticipantResolverContext.getUserProvider().findUser(oldmanager, null)
You might also need to return a vector of users (I do not know which is needed but from your error - just returning the user should work):
Vector<User> approvers = new Vector<User>()
approvers.add(oldmanageruser)