I can confirm that this seems to be an issue with 2.518: Once you add a new element in thetype > Edit Elements, save the changes, and go to assign it to a layout position, the Core Elements are not shown anymore.
We passed this on to the developers, but it's a holiday and I am not sure when this will be taken care of.
To fix the symptom (as oppsode to the cause) without needing to revert to a backup please try this as a workaround:
Access your site via FTP.
Use a text editor capable of UTF-8 encoding and open
media/zoo/applications/[yourApp]/types/[theTypeYouEdited].config
Example: media/zoo/applications/business/types/employee.config
Make a backup copy of this file! (just for safety purposes)
Now look for this whole block of core elements
"_itemaccess": {
"access": "1",
"type": "itemaccess"
},
"_itemauthor": {
"access": "1",
"type": "itemauthor"
},
"_itemcategory": {
"access": "1",
"type": "itemcategory"
},
"_itemcommentslink": {
"access": "1",
"type": "itemcommentslink"
},
"_itemcreated": {
"access": "1",
"type": "itemcreated"
},
"_itemedit": {
"access": "1",
"type": "itemedit"
},
"_itemfrontpage": {
"access": "1",
"type": "itemfrontpage"
},
"_itemhits": {
"access": "1",
"type": "itemhits"
},
"_itemlink": {
"access": "1",
"type": "itemlink"
},
"_itemmodified": {
"access": "1",
"type": "itemmodified"
},
"_itemname": {
"access": "1",
"type": "itemname"
},
"_itemprint": {
"access": "1",
"type": "itemprint"
},
"_itempublish_down": {
"access": "1",
"type": "itempublish_down"
},
"_itempublish_up": {
"access": "1",
"type": "itempublish_up"
},
"_itemsearchable": {
"access": "1",
"type": "itemsearchable"
},
"_itemstate": {
"access": "1",
"type": "itemstate"
},
"_itemtag": {
"access": "1",
"type": "itemtag"
}
and delete the whole block
IMPORTANT:
Leave the last two closing } that follow after the block I quoted untouched! The file should then end in exactly
},
}
}
Now also remove the comma after the third } from the bottom - the one that was immediately before the block to be deleted - so the ending becomes:
}
}
}
Save the changes, close the file.
When assigning elements to positions the Core Elements should be available again.
Thank you