You can't exactly see, what's happening because from this POV it's behind a curtain.
A Youtube comment from a front row concert goer gives testimony:
(google translate)
I was there, standing right in front of the stage!! And I saw everything! First, Alen, the guitarist, was injured. He came back on stage after a short break at the edge of the stage with a head wound, and blood was running down his face. I know him, he made a movement, and he just gestured to see what had happened, vaguely indicating that he had fallen. I thought he only had that head wound, but falling on your face or head is never harmless (I had an accident like that myself). After a short time, they continued playing, and at some point, Peter also went to the edge of the stage and fell directly into the hole and was practically lying under the stage. There was apparently an unsecured hole on the side of the stage, and a curtain was hanging over it. So, if you thought the stage continued and kept walking, you'd step into thin air and fall about 1.5 meters. It was a shock for all of us. Peter didn't move at first either. And I thought something serious had happened, since I also work in healthcare. Another woman told me that he screamed then too; it's possible he briefly lost consciousness. Then five paramedics arrived and used the special sliding board used for back injuries to get him out, and he received a crutch. He was conscious then, as he was still holding up his metal fingers, but his eyes were closed. Both Alen and Peter were taken to the hospital by ambulance. Alen apparently has severe rib contusions, and I think the cut above his eyes was probably glued or stitched. I don't know if he also has a concussion, but they both certainly do. Peter has severe rib fractures, which is no joke. But it looked bad when he was being taken away, and according to people, he couldn't move at first. ... Unfortunately, no privacy screen was provided while they were working on Peter. Normally, that should be done to protect the privacy of the accident victim. It was definitely difficult to get him out from under the stage. I think his further treatment was then done in the ambulance. Alen has probably stayed for observation... Unless something else has come up, but the statement is from today. How long it will take for Peter... No idea, but let's hope he makes a full recovery. Because falling about 1.5 meters, you could really have been killed. I know patients with broken necks from that. ... But I think something will come of it. Let's hope Peter makes a full recovery.