From the results I get from querying for 'custom field unindentified error has occurred', this is quite a common problem.
I followed many wild goose chases from repairing the database to deactivating plugins.
The solution is actually understanding that a custom field is a field that you have to enter a value for.
So I should select the field and then enter the value for the field. In this screenshot, I chose the 'private' field and entered the 'yes' value, then clicked on Add Custom Field.
This is how a successfully entered Custom Field post look like.