No access to the articles anymore/Vantage Child? Gutemberg? CSS?

[ChristinePlume]

I am trying to help a friend with his website based on WordPress.
For 1 month approximately, he can neither

• publish a new article
• go back to the previous articles and modify them.
  When you click on the menu to watch any article, nothing appears but a white screen with an error (see my screenshot).
  He uses the block editor. I modified in order that the editor was the classic editor: then we can click on the articles but then we get only the title and no content anymore.
  What's the problem? What do we have to do?
  He uses Gutemberg. I disabled it but it doesn't modify anything.
  I tried another theme than Vantage Child but the problem remains.
  I checked the website health and... there is no problem found...
  

...

Many thanks for your help...

[talldan]

@ChristinePlume Please could you copy the error and paste it here.

Have you tried deactivating other plugins?

[ChristinePlume]

Yes I did. It seems now, after having checked everything for 15 days and tried many things, including reinstalling WordPress, that the hoster itself (Nuxit) made a mistake. If it's useful, I can post the informations concerning the way we will have solved the problem, when it's done.