I have the same issue as this. It's happened a few times but turning the unit on and off normally resolved the problem. However, mine has been stuck with -28 since Saturday night and I wasn't able to use it. Tried rebooting in safe mode and I've been able to update the firmware but still stuck on the loading page with that error code in the corner. Opened up a ticket but not holding out too much hope as I haven't found a definitive reason or resolution to this on any forum. I've spoken to the shop I purchased it from and they are arranging to collect it and send it away under warranty. Not sure what else to do. Shame, I thought Line 6 would have had an answer to what this issue is, why it happens and what you need to do to fix it but obviously not.
