ISPConfig 3.1 panel internal server error after updating php-fpm php version of another website

Hello,
I have updated PHP version of a website from default (PHP 5) to 7.4 (PHP-FPM 7.4) and then after few minutes ISP Config 3.1 panel stopped working & showing 500 internal server error. (May be cron executed at that time)
error_log showing:
Sun Jul 26 12:05:32.209034 2020] [fcgid:warn] [pid 6293] (104)Connection reset by peer: [client 150.242.67.135:1750] mod_fcgid: error reading data from FastCGI server
[Sun Jul 26 12:05:32.209160 2020] [core:error] [pid 6293] [client 150.242.67.135:1750] End of script output before headers: index.php
[Sun Jul 26 12:21:20.319373 2020] [fcgid:warn] [pid 10932] (104)Connection reset by peer: [client 150.242.67.135:2632] mod_fcgid: error reading data from FastCGI server
[Sun Jul 26 12:21:20.319496 2020] [core:error] [pid 10932] [client 150.242.67.135:2632] End of script output before headers: index.php
[Sun Jul 26 12:22:12.727641 2020] [mpm_prefork:notice] [pid 28341] AH00170: caught SIGWINCH, shutting down gracefully
[Sun Jul 26 12:23:42.296208 2020] [suexec:notice] [pid 1453] AH01232: suEXEC mechanism enabled (wrapper: /usr/sbin/suexec)
[Sun Jul 26 12:23:42.717603 2020] [ssl:error] [pid 1453] AH02217: ssl_stapling_init_cert: can't retrieve issuer certificate! [subject: CN=rnshost.in / issuer: CN=Let's Encrypt Authority X3,O=Let's Encrypt,C=US / serial: 03A6DA7CD8534A39D5058876C0C4A86C14B8 / notbefore: Jul 19 20:28:05 2020 GMT / notafter: Oct 17 20:28:05 2020 GMT]
[Sun Jul 26 12:23:42.717652 2020] [ssl:error] [pid 1453] AH02235: Unable to configure server certificate for stapling
[Sun Jul 26 12:23:42.718922 2020] [ssl:warn] [pid 1453] AH02292: Init: Name-based SSL virtual hosts only work for clients with TLS server name indication support (RFC 4366)
[Sun Jul 26 12:23:42.827439 2020] [alias:warn] [pid 1453] AH00671: The ScriptAlias directive in /etc/httpd/conf.d/mailman.conf at line 6 will probably never match because it overlaps an earlier ScriptAlias.
[Sun Jul 26 12:23:42.836973 2020] [lbmethod_heartbeat:notice] [pid 1453] AH02282: No slotmem from mod_heartmonitor
[Sun Jul 26 12:23:42.842169 2020] [ssl:error] [pid 1453] AH02217: ssl_stapling_init_cert: can't retrieve issuer certificate! [subject: CN=rnshost.in / issuer: CN=Let's Encrypt Authority X3,O=Let's Encrypt,C=US / serial: 03A6DA7CD8534A39D5058876C0C4A86C14B8 / notbefore: Jul 19 20:28:05 2020 GMT / notafter: Oct 17 20:28:05 2020 GMT]
[Sun Jul 26 12:23:42.842197 2020] [ssl:error] [pid 1453] AH02235: Unable to configure server certificate for stapling
[Sun Jul 26 12:23:42.843233 2020] [ssl:warn] [pid 1453] AH02292: Init: Name-based SSL virtual hosts only work for clients with TLS server name indication support (RFC 4366)
[Sun Jul 26 12:23:43.249364 2020] [:notice] [pid 1453] mod_python: Creating 8 session mutexes based on 256 max processes and 0 max threads.
[Sun Jul 26 12:23:43.249395 2020] [:notice] [pid 1453] mod_python: using mutex_directory /tmp
[Sun Jul 26 12:23:43.275888 2020] [mpm_prefork:notice] [pid 1453] AH00163: Apache/2.4.6 (CentOS) OpenSSL/1.0.2k-fips mod_fcgid/2.3.9 PHP/5.4.16 mod_python/3.5.0- Python/2.7.5 configured -- resuming normal operations
[Sun Jul 26 12:23:43.275937 2020] [core:notice] [pid 1453] AH00094: Command line: '/usr/sbin/httpd -D FOREGROUND'
[Sun Jul 26 12:24:19.191047 2020] [fcgid:warn] [pid 6910] (104)Connection reset by peer: [client 150.242.67.135:2804] mod_fcgid: error reading data from FastCGI server
[Sun Jul 26 12:24:19.191173 2020] [core:error] [pid 6910] [client 150.242.67.135:2804] End of script output before headers: index.php

All my sites are working fine other than ISPConfig panel. Please help me in solving the same.

 

Do this first:
https://www.howtoforge.com/community/threads/please-read-before-posting.58408/

How did you install PHP 7? On a server running ISPConfig the default PHP version must remain the original default of fhe Operating System. There is tutorial on how to install additional PHP versions for website use, for different operating systems.