I think i fixed this issue[1]. I added a patch that replaces the affected "require" calls with "require_once". After applying the patch, xajax loads without errors.
The patch should apply cleanly to release 0.5RC2.
[1] SF-Bug #1856875: https://sourceforge.net/tracker2/?func=detail&aid=1856875&group_id=139736&atid=744314
Patch for 1856875, applies to 0.5RC2