Same here. I wound up finding one preset that was freezing the entire unit. I don't have a lot of presets, so it was a little easier to pin point. I did everything from resetting to factory, and restoring from backup, rebuilding the presets to the latest OS, ETC, ETC..even rolled back to 3.11, and it was all the same. As soon as I hit that one preset..the entire unit froze up.
The way I found it was by performing the factory reset, restoring from backup, and not selecting any presets. Exported all the user presets that I had made to my drive. Then did another factory reset, and imported them back one at a time, until the culprit was revealed.
Contacted support, and sent the tech my backup and the one isolated preset, and it did the same thing to his Helix. They tried to figure out what the problem was, but couldn't figure it out, and said, "well hopefully the next update will fix that"...UGH...it's my go to preset, but as long as it's not installed, my Helix works as normal.
3.15 seems to have some bugs that need to be ironed out. I was told another update is coming soon.