If someone in the future stumbles upon the same problem, here is what we found.
Our custom
usergroup ('editor') was given acces to a specific
resourcegroup ('editorDocuments') there weren't any documents in this group yet.
This was done with an acces-policy '
load, list view'. When we changed this to '
Object' the problem was gone.
What happend was this; due to a bug in MODX (
http://tracker.modx.com/issues/4918 ) where the tree can only be sorted when the usr has SAVE permission on ALL resources in the tree.
The weird part in our case was that we had no documents in the affected resourcegroup. the mere fact that it existed was enough to block SAVE permission somewhere, thus we couldn't re-order the tree.
Changing the menuindex manualy did work. Which is illogical because you would think that also in that case several resources needed to be adjusted
Rdg