WordPress website hosted in Plesk is not accessible with a 500 or 503 error

CONFIG_TEXT: There has been a critical error on this website. Please check your site admin email inbox for instructions.

In Plesk for Linux, the following errors may be present in /var/www/vhosts/example.com/logs/error_log:

CONFIG_TEXT: Fatal error: Call to undefined function themesflat_map_icons() in /var/www/vhosts/example.com/wp-content/plugins/themesflat/includes/shortcodes/iconbox.php on line 22

CONFIG_TEXT: PHP Fatal error: Uncaught Error: Call to undefined method EPL_Property_Meta::ang_get_price_stiker() in /var/www/vhosts/example.com/httpdocs/wp-content/plugins/ANG-Esta-make-up/libs/functs/ang-functions.php:211

CONFIG_TEXT: [proxy_fcgi:error] [pid 53999] [client 203.0.113.2:63871] AH01067: Failed to read FastCGI header
[proxy_fcgi:error] [pid 53999] (104)Connection reset by peer: [client 203.0.113.2:63871] AH01075: Error dispatching request to :

CONFIG_TEXT: [proxy_fcgi:error] [pid 16330] [client 203.0.113.2:46016] AH01071: Got error 'PHP message: PHP Fatal error: Cannot redeclare bbt_get_audio_shortcode() (previously declared in /var/www/vhosts/example.com/httpdocs/wp-content/plugins/bbt_fw_plugin/helpers.php:139) in /var/www/vhosts/example.com/httpdocs/wp-content/themes/Wordie/functions.php on line 627\n', referer: http://www.example.com/wp-admin/

CONFIG_TEXT: [warn] [client 203.0.113.2] (104)Connection reset by peer: mod_fcgid: error reading data from FastCGI server, referer: http://example.com
[error] [client 203.0.113.2] Premature end of script headers: index.php, referer: http://example.com
[warn] [client 203.0.113.2] mod_fcgid: can't apply process slot for /var/www/cgi-bin/cgi_wrapper/cgi_wrapper, referer: http://example.com

CONFIG_TEXT: [fcgid:warn] [pid 24533] [client 203.0.113.2:42388] mod_fcgid: stderr: PHP Fatal error: Uncaught Error: Cannot call constructor in /var/www/vhosts/example.com/httpdocs/wp-content/plugins/webpen.de-delivery-by-postcode/wppizza-delivery-by-postcode.php:67

CONFIG_TEXT: mod_fcgid: read data timeout in 45 seconds, referer: https://example.com/contact-us/
Premature end of script headers: index.php, referer: https://example.com/contact-us/

CONFIG_TEXT: (70007)The timeout specified has expired:

In Plesk for Linux, FPM error logs /var/log/plesk-phpXX-fpm/error.log could show the following error:

CONFIG_TEXT: WARNING: [pool example.com] child 50974 exited on signal 11 (SIGSEGV - core dumped) after 0.943055 seconds from start

CONFIG_TEXT: failed to open stream: HTTP request failed!

In Plesk for Windows, the following error may be present in Event viewer:

CONFIG_TEXT: Faulting application name: php-cgi.exe, version: 7.3.1.0, time stamp: 0x5c36749f
Faulting module name: php7.dll, version: 7.3.1.0, time stamp: 0x5c367e24
Exception code: 0xc000001d
Fault offset: 0x00000000002000a2
Faulting process id: 0xd070
Faulting application start time: 0x01d4adcdde6c1eb0
Faulting application path: C:\Program Files (x86)\Parallels\Plesk\Additional\PleskPHP73\php-cgi.exe
Faulting module path: C:\Program Files (x86)\Parallels\Plesk\Additional\PleskPHP73\php7.dll
Report Id: 27a3d8b9-19c1-11e9-814e-00163e558f16
Faulting package full name:
Faulting package-relative application ID:

In Plesk for Windows, Process Monitor shows the lasts loaded items by php-cgi.exe process were some files from a WordPress plugin.