The namespace used is "wikiname" and the expected namespace should be "wiki:wikiname".
XAR extension installation are broken, and fails.
JAR extension are installed but not in the subwiki, but in a 'wikiname' namespace. I was unable to determine if these are effectively installed.
Note that the UI is confusing, and that extension installed anywhere else are show in the installed extension list with an install button.
Installing from the global wiki, using the advanced installation seems to works properly if you specify a namespace in the form 'wiki:wikiname'.