I did the same (expect checking if the installer corrupted the installation medium again) for all of the countless attempts. I either freshly download the .iso or used an .iso I still have on some other drive, use sha256sum, dd it to my installation USB (exactly as described in the docs including conv=fsync) and then even verify the installation medium by using dd to pipe the newly created USB to sha256sum also in the very same way described in the docs. I boot from the double verified USB and the built-in verification fails. In my first attempts, I then reverified the USB afterwards with dd and sha256sum and the sum has actually changed. This happens with 4.2.0-rc5 and 4.2.0-rc2. As you can read above, 4.1 doesn’t have this issue nor this one: AppVMs fail to start on 4.2.0 RC-3 - #9 by b34