Look in the DB in the modx_site_content table, and if you see any resources that have a class_key of mod_resource, change it to modDocument. Also, make sure that no resource is its own parent (IOW, make sure the 'id' field and the 'parent' field don't match).
Some people have also reported problems when the date/timezone system setting doesn't match the system, PHP, and MySQL settings, though none have reported your particular problem, so it does usually have to do with saving categories.
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PLEASE, PLEASE specify the version of MODX you are using . . . PLEASE!
MODX info for everyone:
http://bobsguides.com/modx.html