Copy SQL Server database to same server & decrypt by symmetric key return null
Following worked for me on SQL Server 2008:

OPEN MASTER KEY DECRYPTION BY PASSWORD = 'master_key_password'
ALTER MASTER KEY DROP ENCRYPTION BY SERVICE MASTER KEY;
ALTER MASTER KEY ADD ENCRYPTION BY SERVICE MASTER KEY;
CLOSE MASTER KEY


For me it was essential to drop the encryption first, although the 'add' command should usually do this automatically.