Apache is probably not executing php files properly. Make sure the apache modules are enabled. Screenshots of error: Firefox (something probably cached from before)

Chrome fresh navigation to page Run:

a2enmod php7.3

List of modules that should be enabled

# apachectl -M

Loaded Modules:
 core_module (static)
 so_module (static)
 watchdog_module (static)
 http_module (static)
 log_config_module (static)
 logio_module (static)
 version_module (static)
 unixd_module (static)
 access_compat_module (shared)
 alias_module (shared)
 auth_basic_module (shared)
 authn_core_module (shared)
 authn_file_module (shared)
 authz_core_module (shared)
 authz_host_module (shared)
 authz_user_module (shared)
 autoindex_module (shared)
 cgi_module (shared)
 cgid_module (shared)
 deflate_module (shared)
 dir_module (shared)
 env_module (shared)
 filter_module (shared)
 mime_module (shared)
 mpm_prefork_module (shared)
 negotiation_module (shared)
 php7_module (shared)
 reqtimeout_module (shared)
 rewrite_module (shared)
 setenvif_module (shared)
 status_module (shared)

apt install libapache2-mod-php7.4

• Debian 10 (after upgrade from Debian 9)

When accessing graphs in url nagiosdomain.com/nagiosgraph/cgi-bin/showhost.cgi?host=localhost you get an error

JavaScript is disabled.
nagiosgraph.js is not installed or wrong version.

The issue could be in rrd filename (/usr/local/nagiosgraph/var/rrd) encoding the '“' character as ”%22“ which confuses javascript. Example of filename

Check%20www.karl.com___%7C%20isAlreadySelected%28panel%2C%20selectedValue%29%3B%5Cn%20%20%7D%22%20on%20https%3A%2F%2Fwww.karl.com%2Fde%2Fmantel_cod59141927re.html%20%7CRAUVX.rrd

Simply remove the offending file and reload nagios.

• nagios 4.2.2
• nagiosgraph 1.5.2
• Debian GNU/Linux 11 (bullseye)

• https://www.techrepublic.com/article/how-to-fix-apache-2-not-executing-php-files/
• https://support.nagios.com/forum/viewtopic.php?f=7&t=40351&p=205190#p205190