Even when you have workflows that work fine when they are fired due to an event (a new item or a change to an existing item), you may receive the error "The data source control failed to execute the insert command" when you start them manually. Usually you start workflows in a SharePoint site manually using the .aspx page located in the same virtual folder as the workflow configuration files.
The error is most likely due to a permissions issue. If the user who is starting the workflow manually doesn’t have the right permissions for all the lists that the workflow touches, they will receive this error. It may not make sense because it isn’t the usual Access Denied error, but the error basically means that the workflow is unable to insert an item into one of the lists it is working with.
I recently saw this where a colleague had taken a site template that was available for download from Microsoft (Windows SharePoint Services 3.0 Application Template: Employee Activities Site — It’s a pretty nicely done and useful template, by the way.) and made some minimal changes to it. We first suspected the changes, but it turned out to be the permissions he had set for the site instead. Once those were fixed, everything worked normally again.