Well, it seems that everything is correct now...
I am not sure why this happened but I can tell you that the only thing that I changed was the grid...
The PC with error obj file had the default grid and units set to meters. I changed it to grid size 0.1, snap 0.05 and accented every 10 lines.
I can't see the reason for this to cause an error, because what was 1m from PC1, was 10m from PC2 (error PC)
I should be able to import 10m boxes to Maxwell, right?
Well, anyway, I am very happy to see that it works so I don't really care what happened.

Thanks for the help