I have fixed this problem. The problem was that I was not following the naming convention for MBeans. If the service class name is TaskTimersStartupService then the Mbean interface must be named TaskTimersStartupServiceMBean. This information does not seem to be well documented. I hope this helps other people.
I believe it's part of the spec for JMX. You can also specify the interface class you want to use, if you don't follow the guidelines, by simply indicating it in the -service.xml file. Check out the DTD for the attribute name.