This usually happens when a certain WordPress plugin is running up against the PHP Memory Limit.
Check your error_log file for any error messages that confirm this:
[Mon Aug 18 17:36:30 2021] [error] [client 18.104.22.168] PHP Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 7680 bytes) in /var/www/index.php on line 18
Edit the file
/etc/php7/apache2/php.ini and increase the memory limit:
; Maximum amount of memory a script may consume (128MB) ; http://php.net/memory-limit memory_limit = 128M
Increase the limit and then restart Apache.
service apache2 restart
I also suggest that you should audit your plugins to see what’s using the memory.