Those lines are called stitching and are a screen artifact from flattening the transparency.
What version of ID are you using? The HQ Print preset has not flattened transparency for quit e few versions now.
I use the latest ID Creative Cloud version. I'll check the options if I find something about the transparency flattening.
Thanks for the pointer!
my mistake. I still had PDF/X-3 standard active for the HQ Print preset. That flattened transparencies resulting in the stitching lines.
Thank you for your help!
Your best option would be PDF/X-4