With some qualified help outside of this forum / the laytheme support I could fix the bug. Thought i ll leave the solution here, if anyone else has this problem or maybe @arminunruh @mariusjopen want to change the settings, so this error will be prevented in the future for other users:
i had the setting "carousel size" set to "fixed height", while i had the "slide setting" enabled to "show multiple slides" and "slides width: 90%"
this combination of fixed height and slides width seems to have caused the issue.
solution:
i now use custom ratio instead of fixed height, and set slides width to "auto".
Screenshot shows the now working set up (custom mobile layout)