First of all, a big thank you to @Scherbula, who knew that this tunnel existed, then a big thank you to @Sonni_SP, who knew where it was, and last but not least, a big thank you to Karl Reitmeier, who gave me the following information:
About 1.5 kilometers after the Höll - Lísková (Haselbach) border crossing, there is an entrance to an old mine on the left side, somewhat hidden by the vegetation in the rock. During the communist era, uranium rock was mined there. The entrance to the mine, which was in the restricted area, was covered with blankets so people couldn't see what was happening. The rock was therefore also removed via the north side and not towards the road. However, it is said to have turned out that the uranium rock was not productive enough and then the mining was finally stopped. I don't know how long the tunnel is. Maybe 20 meters. The entrance has now been fitted with a new grate after the old one broke. When the grating was broken, you could go a few meters into the tunnel, in which there is always some water. The water is always dripping in the entrance area and forms interesting structures there in winter when temperatures are below zero for a long time.