I finally found a fix to the "Target not set" issue when dragging and dropping in the tree:
I updated to 2.5.1pl using UpgradeMODX extra and then deleted the manager folder, which had a custom name, and then replaced it with a fresh one from the 2.5.1pl zip
I wasn't able to upgrade incrementally on each major release, so I checked to make sure I didn't have an missing DB Tables as Bobray suggests:
http://bobsguides.com/blog.html/2016/06/27/missing-db-fields-(unknown-column)/
:-)