From what you describe I guess that another app already uses the root-context "/". This is not the case after a fresh Glassfish install. You can see which contexts are used by which webapps on the "Web Applications" page of Glassfish's admin console.
For the root context I suggest not setting the "Default web module"-drop down box. If you use the root-context your app is automatically treated as if it were the default webapp for any requests that do not match another context. Only webapps that use another context ("/someContext") but shall be called when using a url without context ("like
http://www.somedomain.org") need to be configure as the default webapp.
Also a restart is only needed when you change the "Default web module"-drop down box. Another reason not to use this.
But if as a result of previous configuration changes your app is already the default web module you cannot undeploy it. So maybe this is at the root of your undeploy/redeploy problem.
Finally you do not need to enter anything into the "Context Root" input field. It is not mandatory. You can leave it empty and it uses the value of the sun-web.xml. At least this is true for v2UR1.
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