Can you give a full validate with PHP version.
Here are some things you can go after and can try if you have not already done so.
restart LibreNMS server or systemctl restart apache2.
reinstall PHP according to librenms install the correct ubuntu version! https://docs.librenms.org/Installation/
check date /etc/php/7.X/cli/php.ini and /etc/php/7.X/apache2/php.ini See example below (X is version)
[Date]
; Defines the default time zone used by the date functions
; http://php.net/date.timezone
date.timezone = "Europe / Amsterdam"
hopefully you can do something with this and you can solve the problem. These are the steps I did last time.