Can’t edit or delete page

I have a simple page that was created by the wpforo plugin

it has a title, and then a classic block which has the shortcode

[wpfofo] in it.

If I try and update I get

Updating failed. The response is not a valid JSON response.

If I try and delete the classic block and update – I get the same
Updating failed. The response is not a valid JSON response.
together with
The backup of this post in your browser is different from the version below.Restore the backup

If I exit the page and in the/wp-admin/edit.php?post_type=page
page I try and trash I get

Error in moving the item to Trash.

so I cannot edit or delete this page – I am stuck with it – help please

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