Pentax k5 sensor issue

it was a long time since last time I had the same issue. So far I thought the problem was related to sensor overheating, in fact usually it is enough to wait some minutes to solve the issue.
However this time I could not take one shot right. I am sure you can understand my frustration.

I presume you're referring to the reddish stripes bottom right?

If it's a camera fault it's unlikely to go away without a repair, but have you eliminated flare as a possible cause?

if it was a camera fault should be reproducible right? It is not.

It is not even a flare, because it happened only with the long exposure and the ND filter on.

I have used this ND filter dozen of times and this I have seen this issue just once before.

here the situation is more severe, as you can see it happens only on the underexposed area

Are you covering the viewfinder on long exposures to prevent light leakage?

frak, that's what I forget everytime! Do you think it could be it?

no one else experienced the same issue? I've already contacted SRS, but I have a strong suspect that Steve is right.

They are very straight but not horizontal. The light is split as if being refracted through a lens or prism.

I don't know if it is sensor ... light leakage through the viewfinder might be possible or maybe a reflection off the filter frame.

It does look more like light leakage than anything. You didn't mention the long exposure in your original post!

Looking at the angle of the sun, I wonder if direct sun was shining into the viewfinder?

Looking at that I would say there is a 99.9999999999% probability that it is viewfinder light leakage due to the long exposure. Looks just like it, not an issue.