Ubuntu 20.04 Server install from USB - Sorry there was a problem completing the installation

Solution 1:

I downloaded Universal-USB-Installer-1.9.9.0.exe and that successfully converted the 20.04 server ISO into a bootable USB stick. In step 1 I selected Try Unlisted Linux ISO from the bottom of the drop-down list then browsed for the 20.04 ISO and from there it was plain sailing.

_{Answer moved here from OP's question.}

Solution 2:

For me (on an unbranded PC with Celeron 3865U) none of the solutions here worked. Tried rufus, unetbootbin, universal usb. All lead to same problem. Also 18.04 throws the very same error. Eventually I got around this by installing 16.04 and upgrading to 18.04 and finally to 20.04.