performance - "edit page" not loading

This has been happening for the last few days...

1. Click on edit page.
2. The page turns white, the loading bar starts moving and it looks like it's making progress based on this.
3. By about half way along, the loading bar gives up and the page stays on white.

I've looked up other responses to similar things and tried everything I can find:

• Switched the theme to 2019 default
• Bypassed all plugins
• Reset cache

Nothing has made a difference. Has anybody found any alternative solution to this problem?