Processing defect

Hi,
I got this artefact on 6 frames out of 12. I used a jobo daylight tank with 2 films loaded on one reel. The artefact only appears on one film.
Film : HP5+ 120
Dev : DD-X 1+4
Agitation : 15 initial inversions followed by 4 inversions every minutes
Temp : 20°c

Any idea on the cause of this artefact and more to the point on how to avoid it ?

The "ghost" left-center and bright patch lower right appear to be due to camera light leaks; they'd be worst in frames that spent longer in the gate (with bellows extended, if the camera is a folder). They could also, possibly, be due to a cracked lid on the daylight processing tank, which might also produce the "rays" in the bottom center to left of the frame if they weren't part of the scene.

If, OTOH, you're talking abou the crescent shaped mark in the lower left quadrant, that's a "murder mark" caused by kinking the film while loading; I've never seen them on 35 mm or sheet film, only 120. Marks like that are caused by pressure activating halide crystals (or causing local triboluminescence which exposes them) when the emulsion is sharply bent. More practice loading will reduce them, but after a couple years of developing regularly I still get them occasionally. With a Jobo or Paterson reel where you can load two films in succession, they're most common when reaching the loading stage where extra force is needed to load into the less used inner section of the spiral. With stainless, they come from kinks induced when "cupping" the film to slide into the spiral to the bend where it engages the wires.

Thanks Donald, I will have to run some tests, your helpful answer will give me some starting point on what to test
The rays are the most annoying things to me, the image I posted is a crop of the original image, the ray is no so huge, but I hate them nonetheless