In the presented situation, is it allowed to make a U-turn?

In the photo you are driving on a dual carriageway outside a city, next to a lorry with a white semi-trailer on the right; on the left there is a round sign with a red border and a crossed-out U-turn arrow - B-23 "no U-turn". Making a U-turn is not allowed here. The answer is: no.

The Regulation on road signs and signals states that sign B-23 "no U-turn" prohibits drivers from making a U-turn up to and including the nearest junction. The sign stands on the left, at a gap in the central reservation, because that is exactly where a U-turn is tempting - it applies, however, to all drivers, because a sign on the left side of the roadway repeats the sign on the right. On a dual carriageway a U-turn is especially dangerous: you have to cross the central reservation and enter a roadway on which vehicles are travelling at high speed, and a long lorry combination will not manage to stop in time. The ban applies even when the gap in the central reservation looks inviting. If you need to change direction, you drive on to a junction where U-turns are allowed, or to sign B-24 "end of no U-turn". Remember also that the Road Traffic Act prohibits U-turns, among other places, in a tunnel, on a bridge, on a flyover, on a motorway and on an expressway.

Legal basis: § 22 para. 5 and § 2 para. 1 of the Regulation on road signs and signals; Art. 22 para. 6 point 1-3 of the Road Traffic Act (Prawo o ruchu drogowym)