Tintarn wrote: Exactly $n$ chords (i.e. diagonals and edges) of a regular $2n$-gon are coloured red, satisfying the following two conditions: (1) Each of the $2n$ vertices occurs exactly once as the endpoint of a red chord. (2) No two red chords have the same length. For which positive integers $n \ge 2$ is this possible? Isn't this basically https://artofproblemsolving.com/community/c6h347454p1862017?

That's correct.

Was that apparent to the PSC? Because that problem (among many more) with its solution was sent to almost everyone preceeding the last Bundesrunde for preparation. So anyone that received that file would have been capable to just copy the solution.

I am pretty sure that it was not on purpose. On the other hand it is not a big surprise either, given that the two competitions (MO and BWM), for historical reasons, comprise two disjoint Problem Selection Committees. It is unfortunate, but at some point probably unavoidable, that such things should happen. Note that, superficially, it is not entirely obvious that the two problems are the same. So even if someone reads through the old MO problems, one might overlook it (in fact, I myself was preparing the 2006 problem for a training camp just a few days before the BWM problem got out, and it took me a while to notice that it is actually the same!). You might also want to read the story and the subsequent discussion of the (perhaps even more shocking) two (or rather just one!) problem(s) USAMO 2014/6 and USA TSTST 2011/3.

Also 2021 Argentina L2 P3!