confluence/spaces/createspace.vm, lines 97-106:

<script type="text/javascript">
function initFormValidation()

{ if($("key")) var spaceKeyValidation = new InputValidation("key", "/ajax/spaceavailable.action"); }

Event.observe(window, 'load', initFormValidation, false);

</script>

The /ajax/ URL needs a context path before it. This can cause problems in some browsers.