I have encountered this multiple times and this time I am determined to get it solved once and for all.

My Joomla sites are running well on my VPS with php7.2 under ubuntu18.04. Recently I was trying to upgrade them to php7.4. After 7.4 is installed and I set the sites to run under it, the websites just show a blank page with only one word on it:"Error". According to my past experience, this usually means something wrong with the Mysql Server, I check it and it is running well and other websites on the same server that run under php7.2 are fine. So I check the error log to see what generates the error, but there is nothing there! Meanwhile, I write a php file with some syntax error and phpinfo(), the syntax error gets recorded in the log file and phpinfo shows the PHP is running under 7.4.

So why the unknown error which causes the page to show just "Error" doesn't get logged?