Reporting Server cannot decrypt the symmetric key, trying to deploy SSRS reports

I am getting the following error when trying to deploy my SSRS reports on our SQL Server 2014 :

The report server cannot decrypt the symmetric key that is used to access sensitive or encrypted data in a report server database.
You must either restore a backup key or delete all encrypted content. (rsReportServerDisabled) Get Online Help Keyset does not exist (Exception from HRESULT: 0x80090016)
The report server cannot decrypt the symmetric key that is used to access sensitive or encrypted data in a report server database. You must either restore a backup key or delete all encrypted content.(rsReportServerDisabled)
Get Online Help Keyset does not exist (Exception from HRESULT: 0x80090016)

This means that the report server installation does not have the encryption key applied that allows it to access encrypted content in the database. If you have a backup of the current encryption key, you can apply it at the report server. I recommend determining what the likely cause is before acting. This is not something that I have found to just happen - there is usually some trigger, as discussed here.