User505987287 posted
I already found the problem.
\Main\Reports
The developer who did the web application put an impersonate line in the web.config under the reports subfolder of the application where the RS reports are called.
That also explained why I’m receiving an affinity related error during the export if I ran the webpool with another user.
I think the webcontrol of RS are executed using the webpool user since the handler is called from the main folder of the webapp not from the subfolder.
So I remove the impersonate line in the web.config under the report subfolder and it fixed the problem.
Now I’m scrutinizing the developers why it is not mentioned on their documentation.

Thanks