I was able to boot without persistence (I had been afraid that I would have to re-"burn" the USB drive without persistence, in order to test this, but found the 3rd GRUB menu option did the trick).
Lo and behold, the "Trust this executable" was checked. I have NO IDEA how it could have become un-checked (as I had no reason to use the right-click menu - my recollection was that I booted the USB drive, and immediately double-clicked on the icon), but it is now clear that that did in fact happen... I appear to have wasted all of our time. I'm sorry about that. -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1801438 Title: "Install Lubuntu 18.10" icon on desktop results in "Execute File" window appearing To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/calamares/+bug/1801438/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs