That's an old version so it is prior to the new/current pricing so you'd have updates included. Possible that whatever issue you are having has long been fixed in an update. Just curious why you are on such an old version of 6. IIRC 6.12.15 is the latest version of unraid 6.
Since the OS is an older version, going to assume the hardware is also older. Possibly a memory error? Just an anecdote but I've seen a bad memory stick have random problems like you describe where it seems fine but after a while will lock up. I confirmed it by running memtest86 and letting it run.