This might catch a few people out.

 

TL;DR, fe2.update.microsoft.com is being decomissioned and if your WSUS server starts failing to update it hasn't yet switched over to sws.update.microsoft.com

 

https://techcommunity.microsoft.com/t5/Configuration-Manager-Blog/WSUS-synchronization-endpoint-being-decommissioned-on-Monday/ba-p/737039