Applied as following way
Do you happen to have a backup of your encryption key? If not, follow these steps:
Open Reporting Services Configuration Manager
Connect to your RS server
Select 'Encryption Keys' from the left pane
Press the 'Delete' button to get rid of your encrypted content
After doing this, you will need to re-set the connection strings and stored credentials on your reports.
Also, I'd be sure to make a backup of the encryption key afterwards.
If you do have a backup, press the 'Restore' button instead of the 'Delete' button.
Hope this is helpful !