More info:
* There are no errors in the PHP error_log file.
* The error is triggered by WordPress executing its cron job, which as I understand happens every so often when a new page view is registered (so not on a schedule).
* My permalinks are: /%category%/%postname%/
* The error in the debug log seems to be triggered by this line (on GitHub). But commenting out that line does not let the operation succeed (which makes sense; otherwise it would be an empty error check).
* As mentioned, my webhost couldn't find errors on their side, suggesting that it is caused by the plugin.