Unable to upload large files via PHP on Plesk server: HTTP request length exceeds MaxRequestLen

Plesk for Linux kb: technical

Applicable to:

When using PHP as FastCGI, if you try to upload a large file, the below error occurs and causes 550 internal server error or 502 Bad Gateway.
If nginx support is enabled, Apache /var/www/vhosts/system/example.com/logs/error_logcontains the following error:

CONFIG_TEXT: mod_fcgid: HTTP request length ... (so far) exceeds MaxRequestLen (1310720)
Nginx /var/www/vhosts/system/example.com/logs/proxy_error_logcontains the error below:

CONFIG_TEXT: upstream prematurely closed connection while reading response header from upstream

The FcgidMaxRequestLen and/or FcgidMaxRequestInMem directives are not large enough.

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Server-wide

Connect to the server via SSH.
Note: if direct SSH access to the server is not possible, contact server administrator for further assistance.
Edit fcgid.conf file which is located in /etc/httpd/conf.d/ (for RHEL based OS) or /etc/apache2/mods-available/ (for Debian based OS).
Set FcgidMaxRequestLen and FcgidMaxRequestInMem with the same values in bytes. For example, 1024 MB corresponds to 1073741824 B.
Restart Apache:

For RHEL based OS

# service httpd restart

For Debian based OS:

# service apache2 restart

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On domain level