electron-flow arrow button stopped working

I'm giving an exam right now. In the past two hours, two students have shown me that when they had the MarvinSketch 14.11.24.0 sketcher window open, and they were drawing a mechanism, at some point it became impossible for them to activate the two-electron electron-flow arrow by pressing the button. The one-electron arrow could be activated, as well as other buttons. I solved the problem by reloading the applet, but I thought you should know this happened. There are about 50 students in the room.

Dear Bob,

I have created an issue to find out the root of your problem. Unfortunately I could not reproduce it. Was there a common property of the two computers? (Operating system. Browser, etc.)

Regards:
Balázs