Never seen this... are you sure you can add a new folder ? I don’t see why you should be able to do so and not add a new document...
One idea that comes to mind is check the modx_site_content table, does the id field have auto_increment set ? I had a problem one time after transfer, for some reason after I imported the DB from local server, no problem except for this detail...
Another is : did you try re-exporting + re-importing your database ? A corrupt export/import could cause this, or corrupt filesystem...
Hope it helps...
PS : Showing your whole path on a public forum might not be a good idea, just a tip as far as security is concerned
