Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Not sure if you have solved this problem or not, but we've been fighting it for the past few days. I believe we have found a solution.
We have a couple of NSA 2400s using failover. To solve the problem, we logged into the firewall, expanded Security Services, clicked on Gateway Anti-Virus, clicked on Configure Gateway AV Settings, then checked "Enable HTTP Byte-Range requests with Gateway AV".
Hope this helps.