1) Are IPB 4.7.16 and PHP 8.3.7 compatible?

2) Where is the error log of the IPB itself stored? The Apache log is empty. Or I need to turn on the display mode for all errors without exception. Which constant is responsible for this?

It’s not officially supported above 8.1.  So 8.2 and 8.3 are out for official support.

For the second issue, if it’s a blank white page, you would look in the PHP error logs of your server. Apache is not the one throwing the error. Or you could obviously change PHP to display error output but that’s something unique to your server config and would require working with your hosting provider. 

Tweaked configurations, delivered missing PHP extensions, etc. Everything works flawlessly, perfectly. Thanks to everyone who responded! 🙃

Glad you got it squared away!!