I just deactivated all the plugins except all in one security, the error message is gone.
So I’ll activate one by one to see which one is causing the compatibility problem.
Okay, I checked everything and the error came back when I activated this plugin “Gutenberg Blocks and Template Library by Neve theme”.
I decided to do another check.
1. If I deactivated your plugin and activated the rest including the Gutenberg blocks plugin, there’s no error.
2. If I activated all the plugins, the error comes back, but if I take the site out of maintenance mode, which it’s currently in (an All In One WP Security & Firewall option), the error goes away.
So this has to do with the site being in maintenance mode using your plugin while the Gutenberg blocks plugin is active.