Hi Jessica,
Have you tried narrowing your search by only selecting a few folders in the setup? Our instance has over 5,000 campaigns (active and inactive) so my report would
attempt to generate then time out. However, when I reduced the number of campaign folders selected, I managed to get the reports to run. I would try narrowing your filters if you have no already and see if you can get it to run. If you still have issues, I'd contact Support.