The script does not skip ads any more.

Unfortunately an update from YouTube made it basically impossible to fix the script. I've updated the script description to reflect this.

That is a sad thing but at least your script is helpful in muting and hiding the advertisements, so I will keep it on.

@fuzetsu, @gummybear
As muting still works, how about instead of (instantly) skipping the ad, you increase the playback speed to 10x or something? This is as close to skipping as you can get without trusted events.
@ojczeo has proposed a change that worked before the update.
I tested it and it still works if you put it outside the if clause:

Add the following line below line 251:
document.querySelector('video').playbackRate = 10;