Various Problems with WP 5.6

I have the following problems with my website:

• When I go to edit a post, I get a blank page. Nothing loads.
• When I click “Quick Edit” on the “Posts” page, the post disappears (although reloading the page brings it back)
• “Select All” no longer works
• The pop out menus on the left (ie Posts –> All posts etc) do not work
• When editing a post, I cannot click any of the options under the “Publish” menu, ie “Edit”

I have of course tried changing the theme and I have deactivated and re-activated all plugins. None of this makes any difference. The only way I can post or edit a post is by installing “Classic Editor” plugin. This is not an ideal solution and only fixes that one problem. Everything else remains.

What else can I try to fix this? Am I missing something obvious?

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