Ole,
I know how discouraging it is to expose a sheet of film and have it ruined due to a light leak. I have had experience with both new and used holders. The new were Fidelity 4X5's I bought about 18 years ago. All still in service, no problems. I have bought additional 20 used 4X5 holders---no problems.

I have bought old Kodak 8X10 wooden holders...all with no problems other then a corner that became unglued and I repaired. I just bought another 12 8X10 Fidelity holders and they are fine. I have thought at times that I had holders that were problematic but later determined that the problems were either in bellows or camera related. The latest was due to pin holes in the corners of an older Deardorff bellows( would show up only intermittantly depending on how the sunlight was striking the camera.

Even my 12X20's are working fine---with no fogging. If it were me I would check the camera with each of the holders installed and darkslides pulled with a light inserted in the camera from the lens board opening---in a dark room. If you have a leak in a holder it will show...it will also show bellows and camera problems. Good luck.