First hints for live boot using an Ubuntu 18.04 live boot USB stick:
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Press ESC during boot to get boot medium menu (when the white square appears right after switching on or rebooting)
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Choose your USB stick
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If you see an error message like “Missing parameter in configuration file.” and “gfxboot.c32: not a COM32 image boot”:
Press key (you will see the options then
Enter “live” or “live-installation” to boot or install Ubuntu
Then continue as usual.