Warning Message Too Quick to See

When I start Game mode, I get some kind of a warning and a warning sound, too quick to see. It lasts for about 0.01 seconds, then exits out of Game mode. If I then start Game mode again, it starts with no problems.

This is absolutely infuriating, I cannot find out what the hell it is. I tried suppressing all warnings (which I don’t really want to do), and it still happens. Has anyone encountered this?

I tried the ‘pause on error’ in the console - didn’t help. I also tried to stop all warnings from scripts, didn’t help either. It’s really annoying.