That's OK wojbie, you speak English better than I speak any language other than my native language.
I just encountered and error.
I found two turtles apparenlty in a standoff. Turtle One facing the front of the quarry, Turtle Two facing Turtle One, but it is positioned a block below. Turtle two had also mined the block below it. This was roughly in the middle of the edge of Turtle One's sector.
Anyway, one has the error: Mining parallel :22: swarminer :607: attempt to call nil. And the other has the same error, but a different swarminer number. Mining parallel :22: swarminer :706: attempt to call nil.
Upon restart, Turtle One was able to continue mining, but Turtle Two says:
Determining position
Something went wrong.
So Turtle Two apparently got very confused and started digging a different sector than it was supposed to, it may have been traveling to a new sector, because all of the currently unfinished sectors are being actively mined by other turtles. But if it was traveling to a new sector, I have no idea why it was digging below the turtle currently mining that sector. And it was digging down, not up, so it wasn't digging to the surface from down below.
I also wondered if it was maybe a case of the two turtles competing for the same sector as you mentioned previously, but in that case, Turtle two should have "won" the competition because it was lower, but the sector had already had 6 layers removed, and Turtle one had already mined three rows into it's current sector when it apparently encountered Turtle Two.
So I basically have no idea what Turtle Two was doing. I ended up breaking him and re-loading the orders and he's happily mining a new sector.