I'm using google chrome. While testing again, I found a level where it worked just fine (The one with the conveyor belt at the bottom of the screen). I don't understand why it could happen, if you want more details: when I'm activating the start button boxes start to arrive but after a second the boxes seems to have a buggy collision and start going forward/backward a few pixels quickly, making them stay on the same position. Then the others come behind and overflow outside of the rails by collision with the one in front.
I figured out the bug, it was so stupid basically a few walls were in the way of the boxes on the conveyor belt. But I hadn’t noticed because at 60fps they go through, but at 30 they get stopped. I fixed it and asked the help bot in discord. It would be nice to do the patch, but I’m still waiting to see if it’s allowed