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To make it work on Ubuntu 20.04 with PHP 7.4:

Edit /usr/local/pnp4nagios/lib/kohana/system/libraries/Input.php line 57 and replace
if (get_magic_quotes_runtime())
{
set_magic_quotes_runtime(0);
Kohana::log('debug', 'Disable magic_quotes_runtime! It is evil and deprecated: http://php.net/magic_quotes');
}

magic_quotes_gpc is enabled
if (get_magic_quotes_gpc())
{
$this->magic_quotes_gpc = TRUE;
Kohana::log('debug', 'Disable magic_quotes_gpc! It is evil and deprecated: http://php.net/magic_quotes');
}

with
if (version_compare(PHP_VERSION, '5.3.0', 'magic_quotes_gpc = TRUE;
Kohana::log('debug', 'Disable magic_quotes_gpc! It is evil and deprecated: http://php.net/magic_quotes');
}
}

(Credits to https://betanzos.org/gmo/2020/01/13/pnp4nagios-function-get_magic_quotes_runtime-is-deprecated/ )

and edit /usr/local/pnp4nagios/share/application/models/data.php line 979 and replace:
if(sizeof($pages) > 0 ){
with:
if (is_array($pages)) {