I finally managed to figure it out by trying out all combinations I could think of. Seems to me like this thing is too dumb to recognize all valid moves, as don't see why xc5 should be illegal.
~~Anyhow, this thing accepts knight on F4 as the correct answer, written as "Nf4+"~~
Of course it changes when you reload the darn page ๐ซ
Totally, yeah. It always manages to turn it up a notch just when I think it cannot possibly get more annoying.
Spoiler
You get a quicktime event a few steps after the chess puzzle, that "kills" you, if you're not fast enough. Had to start over. Haven't been motivated enough yet to get past the chess puzzle again.