Alan

I got someone to investigate and it appeared that my host put tracking pixel code in too many files – including one called tinymce.min.js. This meant that that broke, so he turned the tracking pixel off and after some fiddling (not sure what) it worked. It then stopped working again on my laptop that evening but by the next day WordPress had been upgraded to v5.1.1. and it’s back working.

I still have the problem that I cannot update my website – I just get white pages when I click the edit option.

It’s been suggested that the upgrade to WordPress v5.1 might be the cause. How do I test a downgrade?

Any other suggestions?

I have now uploaded and activated the correct Health Checker.

I have removed the unactivated plugins and unused themes.

In the Troubleshooting mode, it removed all the active plugins and used the Two Thousand and Nineteen theme. The screen still went white after I tried to edit a page. I have come out of Troubleshooting mode.

What information from the Health Check would help diagnose the problem?

There are two reports:
WP Healthcheck: Your MariaDB version (5.5.59) is compatible with the current WordPress install. However, the WordPress team recommends you upgrade your server to 10.0 or greater.
WP Healthcheck: Your site is not currently using HTTPS. This is insecure and can negatively impact your search engine rankings. Please contact your developer(s) and/or hosting company to enable HTTPS for you as soon as possible! Let’s Encrypt offers free SSL certificates!