Hello Davis,
I ended up finding where the malfunction came from.
It is a parameter that I do not touch skillfully.
Layout is automatically positioned on Sidebar / Content in the first post created after updating to WordPress 5.
It’s by creating a new post test that I understood.
By setting to Content / Sidebar or better to Default, the theme display is restored.
Thank you for your advice that helped me understand.
Good contunuation.
Regards,
Fred