When you employ a Custom Report Type as the basis for a report, you have the ability to select spanning fields that expose fields from the User table when crafting the report (e.g. Created By: Full Name, Last Modified By: Full Name). These fields can then be included as columns or used as filter criteria in the report.
However, if you expose this report to Portal Users, both Customer Portal and Partner Portal Users, these columns will not display nor will the filter criteria used in the report be applied.
The reason for this is because the User table is not accessible to Portal Users. Hence, while they can see the Created By and Last Modified By fields on objects that are exposed to them (these fields are located on the local table of the object), they cannot see the Created By: Full Name or Last Modified By: Full Name fields.
Here is a another article that explains further: Why do Portal Reports receive an 'Insufficient Privileges' error or are missing columns?