OK, that's actually a good thing because it probably means you can debug within PHP.
Look at UploadifyField::refresh(). There may be different refresh() functions for each descendant of UploadifyField, so make sure you check those first. I believe FileUploadField and MultipleFileUploadField, for instance, have their own refresh() handlers.
Find the right refresh() function and just start adding echo statements in the function, leaving your alert(data) statement in there. (console.log() is probably a better option for you, since it will show the contents of the returned object).
Start by putting an echo statement at the first line of the function and work your way down until you figure out why the return is coming back null, and let me know the results!