Hi @Peter Bishop
What is the data type of your parameter? This kind of problem usually happens in the date parameter, the date format does not match. You can refer to this link: https://social.msdn.microsoft.com/Forums/sqlserver/en-US/d126f21b-59e0-48f7-8c9f-b5211fc5d317/ssrs-subscription-strange-quotreport-parameter-is-not-a-valid-valuequot-problem?forum=sqlreportingservices.
Please check that the value of the parameter corresponds to the value in the source table. Check that the parameter data type matches the data type in the database.
Best regards,
Aniya