Server CPU / RAM overload since PHP / NGINX / website update

I am new here and I have registered today to find some real experts which can help me to solve the problem with my Contabo-Server. Of course I have already contacted the Contabo support and asked if they could give me some advice to solve this problem. They just recommended to upgrade the server to an even stronger hardwarer. However, I fear that this will not solve the problem. Certainly this will not solve the root cause of the problem which is obviously software related. In my opinion a 4-core Intel Xenon Linux server (with light-weight Ubuntu 16.04) with 8GB RAM and 16GB swap should be strong enough for hosting a small website.

1. The problem I have since the website update (updated PHP7.0 to v7.4 and NGINX):

After a few hours a process starts and consumes all CPU/RAM resources so that the server gets stuck. I have found out that that a PHP-loop is causing the server overload (acc. htop).

I hope that someone here in this forum can help me to fix this problem. Otherwise my server freezes every 3-7h and then it is not accessible any more (also not via SSH). Then I need to go to the Contabo VPS control pannel and restart it. Afterwards the process begins again (some hours of server idling, then overload by php-cgi7.0 -> server freezes). This is really time consuming and annoying!

2. I have already:

disabled all WordPress plugins which are not required at the moment

disabled all programs which are not needed at the moment

reset all folder and file permissions of the website / user (775 for all folders / 664 for all files)

disabled fail2ban (which also consumes quite much CPU sometimes)

replaced all PHP.ini files with the original ones from the backup

3.Logs:

htop; process with max CPU usage: /usr/bin/perl /usr/bin/php-loop.pl /usr/bin/php.cgi7.0 -b /var/php-nginx/15798927813908.sock/socket

/var/log/virtualmin/investingcrypto.online_access_log:
128.14.134.134 – – (30/Jan/2021:09:29:51 +0100) “GET /webfig/ HTTP/1.1” 404 11454 “-” “Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.113 Safari/537.36”
173.212.226.246 – – (30/Jan/2021:09:32:59 +0100) “POST /wp-cron.php?doing_wp_cron=1611995579.4903509616851806640625 HTTP/1.1” 200 31 “xxx.investingcrypto.online/wp-cron.php?doing_wp_cron=1611995579.4903509616851806640625” “WordPress/5.6; xxx.investingcrypto.online”
90.146.31.227 – – (30/Jan/2021:09:32:59 +0100) “GET / HTTP/1.1” 200 17787 “-” “Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/86.0.4240.112 Safari/537.36”

/var/log/virtualmin/investingcrypto.online_error_log: 2021/01/30 04:39:37 (error) 16769#16769: *254 connect() to unix:/var/run/php/php7.4-fpm.sock failed (111: Connection refused) while connecting to upstream, client: 209.97.148.99, server: investingcrypto.online, request: “GET / HTTP/1.1”, upstream: “fastcgi://unix:/var/run/php/php7.4-fpm.sock:”, host: “investingcrypto.online”