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The brand new 16.04 final does not boot from USB stick:
Missing parameter in configuration file. Keyword: path gfxboot.c32: not a COM32R image boot :
I made a boot stick with Startup Disk Creator on 14.04 LTS on a Lenovo laptop.
I repeated this process on another computer (Gigabyte Brix) with a another 16.04 image (downloaded from another location) and a different USB stick. Also this computer didn't boot.
Upgrading from 14.04 LTS to 16.04 LTS is probably the most common scenario. How on earth did this get through beta testing??
Edit: in 14.04 LTS there was a similar issue: Ubuntu 14.04 LTS live USB boot error