Can you explain a bit more precisely what “The message appears when I edit an event in the Author section” – maybe a screen shot – you can add image links to image sharing services here – so I can try and reproduce.
Ok I can see that you are getting an issue on the Author box but beyond that I can’t reproduce similar in my tests.
We need to eliminate theme or plugin conflicts that my intereact with this.
This may be a plugin or theme conflict. Please attempt to disable all plugins, and use one of the default (Twenty*) themes. If the problem goes away, enable them one by one to identify the source of your troubles.
You can do this without impacting live users, read on …
If you can install plugins, install and activate “Health Check”: https://wordpress.org/plugins/health-check/
It will add some additional features under the menu item under Tools > Site Health.
On its troubleshooting tab, you can Enable Troubleshooting Mode. This will disable all plugins, switch to a standard WordPress theme (if available), allow you to turn your plugins on and off and switch between themes, without affecting normal visitors to your site. This allows you to test for various compatibility issues.
There’s a more detailed description about how to use the Health Check plugin and its Troubleshooting Mode at https://make.wordpress.org/support/handbook/appendix/troubleshooting-using-the-health-check/
Sorry for the late feedback. I’m going back to my stagging site. I just tested again by switching PHP versions from one to another, (i.e. from 7.4 to v8.x) as soon as I switch to 8.x, some elements don’t work anymore, like for example. the date picker, the image insertion, the text of the description which is displayed in white. All the plugins are deactivated, I only use the Twenty Twenty-Three template. You can’t do more native ;-). I don’t understand why you can’t reproduce this phenomenon. If you find a solution, I will be happy to use your plugin.
Finally, I created a new WP instance (php 8.1) and it seems to work, indeed. I think my test site was polluted by all the plugins I tested there. I close this ticket, hoping I won’t need to ask you again.