Unable to edit or delete data driven subscription - rsInternal Error

So what do the log files say?

See if there is any output in Management > SQL Server Logs

Otherwise you may be able to find additional information in the reporting services log file. Its location may be different depending on how it was set up, but here's an example of the file location.

C:\Program Files\Microsoft SQL Server\MSRS10_50.MSSQLSERVER\Reporting Services\LogFiles