In this scenario, is it prohibited to overtake an articulated vehicle?

Ahead of you there is an articulated vehicle - a tractor unit with a semi-trailer carrying an excavator. You are both approaching a junction where traffic lights are working and the light is green. At a junction with controlled traffic the ban on overtaking does not apply, and there is no sign B-25 "no overtaking" nearby. So overtaking the articulated vehicle is not prohibited here.

The Road Traffic Act prohibits overtaking a power-driven vehicle at a junction, but excludes from this ban roundabouts and junctions where traffic is controlled - by traffic lights or by an authorised person. An articulated vehicle itself has no special protection against being overtaken. You perform the manoeuvre while meeting the general conditions: enough space, visibility and checking that nobody behind you has started overtaking you.

Legal basis: Art. 24 para. 1 and para. 7 point 3 of the Road Traffic Act (Prawo o ruchu drogowym)