The X48 service manual is available here (PDF, bios details included in the file). Size is over 10 megs even compressed with 7zip, so it is too big for upload on the forum. https://elektrotanya.com/tascam_x-48_48-track_digital_audio_workstation_sm.pdf/download.html (you'll had to check the captcha then to click on download on the next page). They also have the DM4800 and others service manuals.
Did you ever get the bios for the mkii. I got the same message on the blue screen reinstalled the reboot dvd. Still same thing. Any help will be appreciated