Well, this turned out to be caused by changing the Runtime Name in the deployment wizard. The Verify Upload wizard populates the Name and Runtime Name fields with the .war file name you are uploading. It allows you to change them, but changing the Runtime Name causes this error.
This is pathological. If changing the Runtime Name is going to cause deployment to fail then it should not be allowed.
You can use a different runtime name for that war deployment. Just make sure the runtime name ends with a .war suffix. That should get you past this issue.
Having said that, this indeed is a bug (either the UI shouldn't allow such names or the actual code which identifies a deployment unit as a war deployment, needs a way to handle this). Can you file a JIRA here - JBoss Issue Tracker and add the relevant details and reference this thread in there?