Any options you enter into a gallery's Pro Options text area are written to the gallery's XML file and used by Juicebox when the gallery is displayed. The problem may be with either the flickrSort entry or with Flickr itself but without seeing the exact code you use and your gallery, it will be hard to tell.
Please check the following.
(1) In your email, you say you have tried:
flickrSort = “DATE-POSTED-ACS”
Try using this instead:
flickrSort="DATE-POSTED-ASC"
... using standard double-quotes around the value, no spaces around the '=' character and using ASC instead of ACS.
(2) Make sure that you are not using/displaying a Flickr Set or Group. flickrSort does not work for set and group searches which always use the Flickr set display order. To change the order of images in a Flickr set, check here.
(3) Make sure that your WP-Juicebox has successfully been upgraded to Juicebox-Pro. flickrSort is a Pro configuration only and will be ignored by Juicebox-Lite. Instructions for upgrading WP-Juicebox from Lite to Pro can be found here.
(4) Make sure that you do not also set randomizeImages="TRUE" in your gallery's Pro Options text area.
Hopefully one of the suggestions above will help.
If not, please post back with the URL to your gallery so that I can take a look for myself and help further.