This solution did not work for me on SQL2008. Did the process change for 2008? On the restored database I still get the following error when trying to open the Symmertic Key after successfully "ALTERing" the DMK: An error occurred during decryption. [SQLSTATE 42000] (Error 15466).
Never mind... found the issue:
The solution on the “restored” database copy is to 1) open the DMK 2) ALTER the DMK with DROP ENCRYPTION BY SERVICE MASTER KEY, 3) ALTER it again with ADD ENCRYPTION BY SERVICE MASTER KEY, 4) Close it. Step 3) without Step 2) does not work by itself because SQL thinks it is already encrypted by the SMK and does nothing, reporting success.