Thanks for you quick reply!
The “pause” is just like what I attached here (it is a safe link, picture is stored in my google drive). It is a message appearing on the left of the screen and disappear automatically after 2 seconds or so. I’m not sure what you mentioned is my problem, because from your demostration link I see the experiment stucks there. The demostration link for my experiment is https://farm.pcibex.net/r/aYaGAh/. You will come across the “Pause” screen randomly.