Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
I experienced the same problem with self-administered OpenVPN server I use. After trying many offered solutions (which didn't work), I found this to be an MTU related issue. Setting "mssfix 1400" (and no other MTU related parameters) on both client and server side of OpenVPN solved the problem and restored Office365 connectivity.