Seems to be a bug in Reporting Services.

I came across the same error when trying to alter one of the Subscriptions which was on a Shared Schedule in Reporting Services i.e thru website(http://Servername/Reports/Pages/Folder.aspx)

Deleted the Subscription & tried to recreate it so it could use a Custom Schedule & write the file to Windows Fileshare but it gave me the same error. However, was then able to create the subscription from the Management Studio after connecting to Reporting Services.

Try Browsing thru to that Report under Home--> Reports Folder--> Report Name--> Subscriptions.

You should be able to create/modify/delete subscriptions from here i.e even set schedules,parameters etc.

Regards,

Neeraj