OOH OOH I KNOW THIS!
(Sorry, I didn't see your post about schedule timings before my last reply.)
I think I had the same problem once about storage lag and timeouts. It's a T_DEFAULTS setting: EMAIL_REPORT_FILE_TIMEOUT. I have mine set to 10 (seconds).
P.S. Here's a thread about the Report Server. P.P.S. Join us in Slack! Info: https://tessituracoders.bitbucket.io/