It does look a bit like booting from the floppy, but I think the Mackie OS Loader v.3.0 (...that's something that is shown when booting from an installation floppy - this is not the same as Mackie OS 3, btw.) also starts up when starting the OS from the harddrive (although then it doesn't usually show this type of message, if things work as they should).
Could you clarify, please, if this happens when booting from the floppy (e.g. with the intention to re-install the harddrive), or is this happening when booting from the harddrive?
To me, it looks like either the medium this is running from got corrupted (i.e. what's on the floppy or harddrive), or possibly some hardware got replaced, that is not compatible with the Mackie OS, or the floppy drive no longer works as it should (not uncommon... old floppy drives often can only read floppies that were written on the same drive... tolerance issues, I guess).
So, IF this is when trying to boot from the floppy, create new floppy disks first, via the downloads available here:
https://www.sonido-7.com/d8b/index.html#Post6Forget about the Mac downloads, unless your Mac is still running OS9 or older.
Use the windows downloads, extract them, and then use the app that got extracted, to write 3 new Mackie OS 5.1 (I'd go with this now, and apply the hack... no reason to still deal with Mackie OS 3, unless you have very little RAM). Then modify the first floppy as described in an earlier post.
Then put the floppy in the D8B rack unit and try again.
If this happened while booting from the harddrive, then either the drive got corrupted, or possibly some hardware may have gotten replaced, that is incompatible with the drivers the Mackie OS tries to load... but IMO, that's unlikely, because pc_dskopen doesn't sound like something that needs a special driver.
You didn't replace any hardware, other than memory or the harddrive, right? (Most other things will need to work with the built-in drivers in the Mackie OS, so the list of devices that will work, is extremely short... and not available as a list, haha).
Anyway... more info, please. Ideally a step-by-step description of how you got to that point.