It's not really an issue with Grav then, sounds like an issue with PHP. Grav doesn't access anything directly, everything goes through PHP. What version of PHP is Grav running on your Ubuntu 20.04 compared to your 18.04 ?

On Ubuntu 20.04 I tried Grav with both PHP 7.3.21 & PHP 7.4.9 and had the same issue. However, Grav ran correctly on Ubuntu 18.04 with PHP 7.4.9. This is all on Apache by the way.

I have just setup a similar environment @jonathankaiser except in my case it was using WSL2 instead of Vagrant. But I had a fresh Ubuntu 20.04 LTS and PHP 7.4.3. I had a bunch of errors too, unfortunately I can't recall if was the same. I know it's a long shot, but maybe the issue is the same I had? My errors were due to Apache permissions. I had to update the owner/group for my Grave files to be the same as what Apache uses www-data.