SQL Reporting Services - Data Sources

Am I missing something?

I recently posted this discussion (Previous) and managed to resolve it as detailed, however I have now hit a similar issue with a Data Source that I can't figure out. It seems (or my report seems) to behave differently depending on the credentials within my data source.

I use integrated credentials in the data source (see below)

And I get output as expected in my report:

I use Stored Credentials in my data source

and get the following error: -

Has anyone seen this behaviour before? It seems directly linked to the credentials used and I have no idea why! Clearly I want to use stored credentials as our standard report users will not be able to use integrated security.

Does anyone have anything I could try to resolve?

Thanks

Glenn