Cannot save post: REST API error 500

Hi,

Thanks for your help previously, it solved a lot.

But now I cannot save any draft pages or post.
I get an error message each time I try.
I guess it is linked to the REST API (below the error I see)

The REST API is one way WordPress, and other applications, communicate with the server. One example is the block editor screen, which relies on this to display, and save, your posts and pages.
The REST API call gave the following unexpected result: (500)
There has been a critical error on your website.

It is clear that there is a problem, but I have no clue what to do to solve it.
Can you let me know what I should look at?

Thanks for your help

Barbara

The page I need help with: [log in to see the link]