When viewing, in Central Administration, the Nintex Content database mapping settings in Nintex Administration, the following error might occur:
 

  
Identify the database name which is deleted from the databases table. 

Select * from dbo.databases. 

Get the DatabaseID

Select * from dbo.storage  where Databaseid = 'DBID'

If you find any, Run the delete command 

delete from dbo.storage where Databaseid = 'DBID'