SYMPTOM:
The following message is displayed in Strategy Web when a Web Reporter user tries to swap the element of a page-by drop down menu when the report has been drilled into a new window:
Job Expired. The results for this report are no longer available
STEPS TO REPRODUCE
Execute a drillable report with an attribute in the page-by field and drill one of the attribute elements into a new window, as shown below:

When trying to select a different attribute element in the Page-by field of the parent report, the 'Job Expired' message is displayed.

CAUSE:
Web Reporters do not have the Web Analyst privilege of 'Web simultaneous execution' privilege. Every time a new report is opened, the previous report is no longer available. This limits the user to one document or report instance that may stay available in memory on the Strategy Intelligence Server for manipulations. As soon as the user drills the parent report, its results will no longer be available for manipulations, and only the drilled report is available on the Strategy Intelligence Server, as shown below:

Another similar behavior caused by the lack of this privilege is explained in the following Strategy Knowledge Base technical note:
KB16746 (KB5600-8X-2773): 'Job Expired The results for this report are no longer available. Please submit the request again' message is received when drilling twice on a source report with the option to 'Open drill results in a new window' checked in Strategy Web 8.x
ACTION:
In order to be able to drill a second time on the same source report, the user will need to have 'Web simultaneous execution' privilege which is part of the Web Analyst licence.