Selecting a content item, taxonomy, or another type of data generates the following error: Conversion from type 'DBNull' to type 'Long' is not valid."
This issue can be resolved by forcing the content into an 'A' (appreoved) status by executing the following script on the database:
update content set content_status = 'A' where content_id = (enter contentID here)
and content_language = (enter content language here)
As with any database changes, create a backup of your database before executing the script.