Just to confirm, the logs you reviewed are: C:\Program Files\Microsoft SQL Server\MSRS12.SQL2014\Reporting Services\LogFiles (for SQL Server 2014). If memory serves, this can be configured during install time, so that folder MAY not be accurate for your system.
If not, I'd check that log. I'd also check the server logs and sql agent job logs.
BUT the error you presented tells you what was wrong - the subscription contains parameter values that are not valid. So I would check the parameters in the subscription to ensure that they will ALWAYS be valid for the report. Next, I would recreate the environment where things failed in a test environment that is from the exact time things failed with the report subscription and run the report with data from that time. It is possible that the report succeeds at 10:00 AM but at 9:00 AM that parameter isn't a valid parameter for the report.
The above is all just my opinion on what you should do.
As with all advice you find on a random internet forum - you shouldn't blindly follow it. Always test on a test server to see if there is negative side effects before making changes to live!
I recommend you NEVER run "random code" you found online on any system you care about UNLESS you understand and can verify the code OR you don't care if the code trashes your system.