Is it allowed to reverse a vehicle in a place marked with this information sign?

In the video you are driving along a two-way street with a double solid line in a city behind a dark car and entering a short tunnel under an embankment; on the right there is the blue sign D-37 "tunnel". Reversing a vehicle is not allowed in a place marked with this sign. The answer is: no.

The Regulation on road signs and signals states that sign D-37 "tunnel" marks the entrance to a tunnel, and the exit from it is marked by sign D-38 "end of tunnel"; the Road Traffic Act prohibits reversing a vehicle in a tunnel, on a bridge, on a flyover, on a motorway or on an expressway. In a tunnel there is no shoulder or room for others to go around a reversing car, and in the artificial lighting and noise, drivers behind will notice late that the vehicle in front of them is moving in the opposite direction. That is why other restrictions also apply in a tunnel: a ban on stopping and a ban on U-turns, and you drive with dipped headlights, not daytime running lights. If you miss an exit or make a mistake in a tunnel, you drive to its end and make a U-turn at the nearest junction where this is allowed; if your car breaks down, you switch on the hazard warning lights, but you do not reverse to the entrance.

Legal basis: § 56 para. 1 of the Regulation on road signs and signals; Art. 23 para. 2, Art. 22 para. 6 point 1 and Art. 49 para. 1 point 3 of the Road Traffic Act (Prawo o ruchu drogowym)