"The website is having techical issues", but no errors in any logs

Hello,

As my site was still on PHP 5.6, I've made the jump to PHP 7.3 and be ready for the future (and for increased speed and security mostly).

However, since the upgrade, I regularly receive errors on the site, both front and back-end. I've checked my php error logs, and I've set up wp_debug and wp_debug log, but all remain empty. Nothing comes into the logs when I envoke the error.

The error itself is also not clear… As in: it happens when I do a "new page", or when I go to the "themes" page. Not when I try to edit a post for instance. Not really consistent.

I've tried disabling all plugins, and enabling them again one by one, but there is also no consistency in that, sometimes it fails again, sometimes it doesn't. I'm currently on a 'as bare as possible' setup: divi theme, WPML basic multilanguage plugin, WPMU Defender, Smartcrawl and Hummingbird. All are the latest versions…

Thanks for any possible help!

Bjorn