I just realized, that mine situation isn’t as same as yours, but maybe they are connected... I had js syntax problems in all modx revolution versions ;(
For all, who found this post and still searching for answer i've got a tip: Check all your AJAX requests. On some servers (especially free ones) there's some JS code (using for eg. statistics purpouses) is adding to ALL requests (including AJAX) and causes this type of problems.
