I should have said how to reproduce it, sorry - I clicked the blue warning triangle in the bottom left corner, then “Update now” then the password of the main admin user.
The instructions say to run the cron as the web user, but I tried running the cron as root anyway and the error message changed to a long string like this:
500: Cannot update system because 64c08d… is not writable
I did try manually updating by unzipping the latest version and one of the puzzling things is I still see an update notice, but only about the web app.
I’m not sure how the database gets updated if I just unzip things (newbie here, trying it out for the first time so a certain amount of ignorance is to be expected!).