@j00k4 I suppose you use FOG 1.5.8, right? When we released this version Ubuntu 20.04 wasn’t out yet and while I have tried to get it right, Ubuntu decided to switch to a different PHP version just a few weeks before the final 20.04 release.
So please download the latest release candidate version of FOG and give that a try: https://github.com/FOGProject/fogproject/archive/1.5.9-RC1.zip