SFB Hybrid move accounts using onmicrosoft.com account

ok, problem solved :). It appeared to be one of those RTFM things you don't pay too much attention to especially when you are in a rush. The msft doc https://docs.microsoft.com/en-us/skypeforbusiness/skype-for-business-hybrid-solutions/deploy-hybrid-connectivity/move-users-from-skype-for-business-online-to-on-premises says :

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select and copy the URL in the address bar up to lync.com. An example URL looks similar to the following:
https://webdir0a.online.lync.com/lscp/?language=en-US&tenantID=

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and that is what we did, missing the next step that says "and now replace webdir with admin"

Now the funny part is that actually the webdir url actually worked when we used the sync on-prem csadmin account, and it started failing after we decided to use onmicrosoft.com account to  move users to SFBO.