Odd error

Try deactivating all plugins. If that resolves the issue, reactivate each one individually until you find the cause.

If that does not resolve the issue, try switching to the default theme for your version of WordPress to rule-out a theme-specific issue (theme functions can interfere like plugins).

Also try downloading WordPress again, access your server via SFTP or FTP, or a file manager in your hosting account’s control panel, and delete then replace your copies of everything except the wp-config.php file and the /wp-content/ directory with fresh copies from the download. This will effectively replace all of your core files without damaging your content and settings.

Some uploaders tend to be unreliable when overwriting files, so don’t forget to delete the original files before replacing them.

It’s a custom theme but it’s been fine for months. I’ll try all you suggested but is this a major issue or one to ignore? I mean, it’s just the cache file, right?

Offhand I’d say only a couple of plugins are active since the site isn’t open yet. Just checked and deactivated all five.

I checked yesterday’s error log more carefully. Ten times it erred. Every hour or two it happens. The only other errors are people trying to login but no users are in the db.