I got it working !
It seems to be a common issue with an update on april, who seems to have disabled Guest SMB access.
I created an hidden account on Windows with the following bat and reg :
And set the correct credentials on OSMC (first by SSH, didn’t work, then trough the UI).
Is there anything I did wrong, or that I should do to have a better update experience ? It’s the second time, so I’ll be a bit frightened to do it a third time
Or… If it ain’t broke, don’t update it ?
Thanks AndersG anyway, and people involved in this awesomeness.