Hello simerti,
What I suspect is happening is that your seams in your lightmaps are at those corners. What happens with seams is there is essentially a break where the calculation is taking place. This can cause artifacts and the black lines as you see. I see you have tried some of the other troubleshoots already, and I believe your lightmap UV’s are causing an issue.
If you would please provide a screenshot of your UV’s along with your lightmap UV’s I will take a look at those.
Also, I am linking your to our documentation on how to unwrap for lightmaps.