What PHP modules are installed on your system?
On my standard Ubuntu 20.04 there are the following PHP packages installed and i have no problem with NagVis
root@monitoring:~# dpkg -l | grep php
ii php-cgi 2:7.4+75 all server-side, HTML-embedded scripting language (CGI binary) (default)
ii php-cli 2:7.4+75 all command-line interpreter for the PHP scripting language (default)
ii php-common 2:75 all Common files for PHP packages
ii php-gd 2:7.4+75 all GD module for PHP [default]
ii php-json 2:7.4+75 all JSON module for PHP [default]
ii php-pear 1:1.10.9+submodules+notgz-1ubuntu0.20.04.2 all PEAR Base System
ii php-sqlite3 2:7.4+75 all SQLite3 module for PHP [default]
ii php-xml 2:7.4+75 all DOM, SimpleXML, WDDX, XML, and XSL module for PHP [default]
ii php7.4-cgi 7.4.3-4ubuntu2.4 amd64 server-side, HTML-embedded scripting language (CGI binary)
ii php7.4-cli 7.4.3-4ubuntu2.4 amd64 command-line interpreter for the PHP scripting language
ii php7.4-common 7.4.3-4ubuntu2.4 amd64 documentation, examples and common module for PHP
ii php7.4-gd 7.4.3-4ubuntu2.4 amd64 GD module for PHP
ii php7.4-json 7.4.3-4ubuntu2.4 amd64 JSON module for PHP
ii php7.4-opcache 7.4.3-4ubuntu2.4 amd64 Zend OpCache module for PHP
ii php7.4-readline 7.4.3-4ubuntu2.4 amd64 readline module for PHP
ii php7.4-sqlite3 7.4.3-4ubuntu2.4 amd64 SQLite3 module for PHP
ii php7.4-xml 7.4.3-4ubuntu2.4 amd64 DOM, SimpleXML, XML, and XSL module for PHP