Having same issue, high Sierra 10.13.6 on a mid 2011 Mac mini. [br]
So far, I’ve backed up all files to an external hard drive using time machine. Also wiped the OS drive for a fresh install. I’m currently migrating all old files all back to the fresh OS drive. [br]
During this process, the Mac mini has restarted, so I’m guessing a fresh install isn’t the fix for me. [br]
Im also seeing High Sierra is a common factor with the boot issue. I’m going to try and update to a newer os if it’s even possible. If not I’m guessing a hardware issue?