New WordPress isn’t working

We need to rebuild our site (hosted by IONOS) and have just installed a new version of wordpress.  We’re being told it’s installed correctly.  However…

We try to edit a page with Elementor, and

1. it doesn’t load
2. after about 20 seconds, a pop up says we need to enable ‘safe mode’
3. we hit the ‘Enable safe mode’ button
4. a separate pop up says ‘domain.name says an error occurred’
5. we click ‘OK’ and it just reverts to the unloaded Elementor page with the ‘enable safe mode’ pop up.

So…

…we try switching to another installed theme – we just get presented with a blank screen

…we try installing a new theme – we get an error message.

Do we need to uninstall and start again? If so, what’s to stop us facing the same problem? If not, what’s the fix we need?

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