I had an old 6500 that had this code.
Did everything you have done but was still no good.
Then I found out you could suppress the code by setting a dip switch ( can’t remember which one. Sorry).
The machine ran the last year of its contract like this with no issue with colour registration.
And before anyone asks, it was in a print shop with a particularly fussy operator so any issues would have shown up quite quickly.
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