88 Seville - Lost odometer reading. 'ERROR' displayed..

This is about my 1988 Cadillac Seville, 4.5L V8 with the digital display.

A few months ago I started to notice that the odometer mileage was displayed correctly, but once in a while the word 'ERROR' was displayed on the odometer for a fraction of a second, then the the mileage was displayed again..

I didn't pay much attention to it then, but since a few weeks ago all I see is the word 'ERROR' displayed on the same spot where my mileage used to be. Sometimes the mileage is displayed, but only for a millisecond. If you don't look in that direction.. you just missed it!

What is also very interesting, is that the number for the mileage (if you are lucky enough to catch it when it is displayed) doesn't seem to add up any more. I drive about 40 miles every day and sometimes at the end of a day the odometer shows LESS total miles than the previous day. How is it possible to 'lose' miles?!? :)

I would think it might be some sensor problem.., but.. doesn't the odometer and the trip meter use the same sensor??? Please note that the trip meter works perfectly and accurately at any given time! No error displayed there at all. It counts the miles, everything adds up, while the odometer shows junk.. The speed and everything else is as accurate as possible and everything works like the rest of this awesome oldie.

I was told that it might also be the digital speedo cluster.. Or something far more complicated and I don't even remember the name of that part. Sounded extraterrestrial anyway.. :)

Does anyone have an idea what may be wrong with my odometer? It is a great Caddy Seville and it drives like a brand new one at its 278,300 miles (this number is for real, I write down my miles at the end of every week.., but the last readings are from the trip meter, not from the odometer!). My odometer can't be trusted any more, it still shows 277k.

If anyone needs any additional information please let me know and I will provide it.

Many thanks in advance!

PS - I don't know if it helps or not, but here are also the codes reported in diagnosis mode:

E048 HISTORY
B120 HISTORY
B121 HISTORY
B123 HISTORY
B441 HISTORY
B448 HISTORY
B552 HISTORY
B556 CURRENT

Does anyone know what they mean or where I can find something to translate these codes into 'english'?!?

B556 is odometer EE prom error. Check for proper EEprom installation in BCM. If prom is properly installed, no bent pins, then replace prom. If a properly installed EE prom continues to set B556, replace BCM. EE proms are car specific and must be adjusted for mileage. Do not touch prom without first grounding possible static.
The other codes are:
E048= EGR Sys fault
B120= Twilight delay switch pot circuit problem
B121= twilight circuit open/short
B123= panel lights dimmer open/short
B441= cooling fan problem
B448= Low refrigerant pressure
B552= battery / power interrupt
all the above codes are old codes from previous repairs.