Thank you so much for your answer! I didn’t know that the lightmap resolution of the ground/wall actually mattered. I increased the lightmap resolution and it works!
I know that the helmet should be movable but it is for an assignment for university and so creating a lightmap was a requirement for the assignment wether i created a helmet or anything else.
I also have another lightmap where i did not waste all of this space haha. I just re-scaled the lightmap just to make sure that the issue was not caused by overlapping. at first i thought that the issue was caused by overlapping uv’s.