I happened to pause it at 25:52. It's a shot of a foxhole lined with sandbags. When it runs, you see a white flash in the lower left corner. I stopped it at the right frame and I could see what the flash was. It looks like a person that somebody tried to scratch out of the print. You can see his head and the marks from whatever was used to alter the negative. Given that it's old film, and not professionally edited, it may have been a mistake that they forgot to edit out and someone tried to fix it the final print. It looks like they wanted to show the landscape and there was a soldier in the way during part of the shot.
IMO, the editor tried to cut out the part with the soldier in it, but missed a couple of frames.