Hello Bert
I understand what you are saying - but it does seem unlikely that the faulty printer was faulty from new (which if would have had to have been, if the white marker line was installed 3mm off where it should be in the factory) - as otherwise, if this was indeed the fault, it would either have never worked properly from new, or always worked fine from new - it couldn't have been the case that it only worked "sometimes" fresh out of the box when it was new, otherwise it would have been returned by the original purchaser.
Otherwise, as I suspect is probably the case, that the printer worked fine from new, and became faulty at a later point in time.
As I asked about before, the white marker line could not possibly have moved itself during it's lifetime, so this seems to suggest that this can't really be the reason. Isn't it more likely to be some sort of fault with the scanner unit itself (or the drive belt that moves it or something like that) that prevents the scanner unit moving back to exactly where it should do after every sweep? That's the only logical explanation that I can see.
It would also explain how when I experinced the problem - physically moving the scanner unit to the far right of the base of the scanner unit cured the scanner error problem - it's almost as if some grit or other fluff or something like that was preventing it from moving it through it's full cycle of movement, and physically moving it to where it should be released whatever was blocking it and it's been ok ever since. At least, from a non-engineer point of view, that's how it seems....