[Scuderia_Turini]

One thing puzzles me though - why did it only affect one lane when the other one was connected to exactly the same power source?

Might be due to the path that the wiring takes. With my opto-sensors, I always wire a small capacitor (like the suppressor ones on motors) across the sensor wires at the sensor. That helps reduce any "spikes" that come up the wires. Also I use screened audio cable to connect to the parallel port or the Arduino.

Leo