Problem:
I have experienced this same problem... in fact on a current project I lost a complete day of work (I wasn't backing up my DB often enough & my fix was to restore my DB) in attempt to resolve it.
Once I found Radix's answer I realized that was definitely what I had just done (disable all of those items in the navigation menu).
Procedure/Fix:
I then took it one step further, since, in my estimation putting these links in the navigation menu must have been an error (what business do they have there??).
I went to Structure->Menus and clicked the "list links" errr link.
Next I clicked the "edit" link on the Add content sub-menu item.
In the Edit window I selected the ----Content sub-menu of the Configuration menu.
I then verified that clicking Add content from the management menu AND from the Content window brought me to the links I was expecting, and it did.
Drawback:
I found that when adding new content types you have to modify their automated menu link by finding them under the navigation menu & setting their parent menu as ------ Add content
Summary:
Moved the Add content menu to a more logical location (so I could use the navigation menu for my own purposes without losing the ability to add content), however this requires an extra step when creating custom content types.
node/add/page
,node/add/article
etc? – Clive