I have deactivated LiteSpeed Cache and the site is now working perfectly.
I am also having bad ongoing issues with this, with it starting during 2018. I have re-installed the WordPress so many times but it doen’t fix the problem all the time. I have also gone into the problem page and resaved it (not changing the password) and this seems to work for a day or two.
I wish someone out there would explain how to fix this in a simple way! I am not very code-savvy and the site in question is used by lots of other non-techy people, so help! Please!