Just to let you know I figured it out. And the answer was stunningly simpel: I did rename the file before I uploaded it. So instead of “wp-masonry-layout-pro” I called it “masonry plugin” (I like simple names..) This is why certain files couldn’t be found as the path to that files included /wp-masonry-layout-pro/!
So I removed all old files, renamed my original file back to : “wp-masonry-layout-pro” and then uploaded it again. It works totally fine now!
Thanks for helping me to find the direction where to look.