You will need to reset your encryption keys. To do that:
Open Reporting Services Configuration Manager and select 'Encryption keys' tab
Click 'Delete' in 'Delete Encrypted Keys' section:
You will need to reset all connection strings and db credentials for all your data sources.
And deleting the keys will not delete any data on the server. You reports will be fine, however your connection settings (data sources) will be gone. You will have to re create the connection for the reports.
You have to delete the encryption key because something is wrong with the encryption. Some setting must have changed. If you do not have a backup key, you have no other choice. Delete, create a new one, and create new connection strings.
Not a big problem.