I finally got it fixed by using the following command :
Get-AppXPackage -AllUsers |Where-Object {$_.InstallLocation -like "*SystemApps*"} | Foreach {Add-AppxPackage -DisableDevelopmentMode -Register "$($_.InstallLocation)\AppXManifest.xml"}
I thought I had tried that before, but for some reason it "stuck" this time... I hope this lasts for
a while! Thanks for the clue...
Colif :
does this work?
open start menu
right click on picture at top (your user name)
select change account settings
On next page should be a link saying sign in with Microsoft account