Hi, I had this exact same problem. Helix LT was stuck on the "Failed to boot, entered update mode" screen and the only button combo that worked was 6+12. My Line6 updater wasn't seeing that the Helix was plugged in. Thought that it was a USB issue and proceeded to try different ports and computers.
The thing that fixed it for me was putting it into update mode (6+12 on the Helix LT) and then installing HX Edit 2.80. Somehow this installed something that I previously did not have. The Line6 updater was then able to see that the Helix was plugged in and I had absolutely no issues with installing the update.
