You will find references to DefaultDS within files such as login-config.xml and ejb-deployer.xml. Seems to me that DefaultDS is used to persist some internal stuff within the app server.
I had the same problem and in the end I just restored the hsqldb-ds.xml file (which I had deleted since I'm using mysql).
Now I haven't had the time to find out why this is so, or if there is a way to change it.
I faced the same problem. Traced it to the following in mysql-ds.xml file.
This needs to be uncommented/enabled so that the corresponding type-mapping from standardjbosscmp-jdbc.xml is selected.
This fixed the error.