So if I understand correctly, it's working from BIDS but not working after it's deployed?
When you deploy a report that replaces an existing one on the Report Server, any default values that have been set up are NOT overwritten.
So maybe what's happening is that even though you've updated the parameter and it's all working fine in the designer, when you deploy it since the parameter already exists it doesn't update it with a new default value as you require.
To test, after you've deployed an updated version of your report, I would double-check the default values associated with the report through Report Manager. Update the Report Server default value and see if this allows the report to run.
Edit after comment:
Onec a report has been deployed, in the Report Manager website Manage the report and check the Parameters:

You need to check the Default Value setting and anything else that needs to be updated.