While doing a run on a custom map my screen turned white (with still the speedometer, the timer, ecc.) , and I wasn't able to do anything (like turning off full screen) for a few seconds. After reloading the page this error was shown:
Sent another track! I don't know how much hard it actually is, because I got two crazy bounces in one of my first runs and then one of the only jumps to the finish that works, but I think it's ok.