Hi, @Francois
Thank you for posting in Microsoft Q&A forum.
Have we make sure the SMS_SITE_COMPONENT_MANAGER service is running?
And the documentation suggests to run a site reset if we are upgrading the primary site server.
For the bgb issue, we may try to change two files as the blog suggest, then remove and re-add the MP role to see if it helps.
For the reference:
https://social.technet.microsoft.com/Forums/en-US/d8a5e063-b643-4b89-a2b6-c4661d6b5905/failed-to-create-bgbservercontroller-instance?forum=configmanagermigration
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