trying to connect to a Sharepoint list as a data Source but get errors

I'm trying to  access a Sharepoint list as a datasource, we do not have the reporting Service intergrated in to Sharepoint as we need SRS available for other reporting.

but we will eventually call the reports via pages with in Sharepoint....  long story as to why no integration with SPS2013.

here is the error message:

An error occurred when accessing the specified SharePoint list. The connection string might not be valid. Verify that the connection string is correct. (rsSPDataProviderError) Get Online Help

For more information about this error navigate to the report server on the local server machine, or enable remote errors

the report will run from my desktop  but when I upload it to the report server  reports area then that error is thrown....

 what could be the cause of this?