Unfortunately, I cannot figure out what may be causing your issues. Are you getting the "subscription" not found error, or "invalid parameter values" error?

I am currently not using DD subscriptions actively. In fact, I no longer had the SP, function, and table in the SQLDB. To try and track down the problem, I created the SP (from page 5 in this forum), including the fn_split UDF and CustomSubscriptionHistory table. I went to my development Report Server and then created a subscription. I then ran some sample code to change the subscription parameters and save the customized report. Everything worked flawlessly. I changed the parameters and ran it a second time. There were no errors.

Keep in mind I ran this as a DBA (in SQL 2008 Standard Edition). It is possible you have a permissions issue in your DB, although I would expect a different error in that case. It is also possible the Subscription is being automatically deleted after it's run. When you create the new subscription, schedule it for the past and to not run automatically in the future. Hope this helps!