Just to add, I found this post due to the same error when upgrading from Veeam for O365 126.96.36.199-something
When the error popped up, I opened a command prompt, ran "iisreset /stop" (waited for IIS to stop) then went back to the stuck upgrade, clicked 'retry' (upgrade finished successfully), then back to my command prompt and "iisreset /start". All working fine now.
Mike Resseler wrote:Are you running something specific on that IIS server?
I have two websites, binding are...
Default Web Site = 80
VeeamBackup = 9080 (http) & 9443 (https)
running netstat shows...
Proto____Local Address_____Foreign Address_____State