Just to be sure, who created the invite? A German native speaker by chance?
The first page of results when I deliberately google in English “what is a Jour Fixe” are the following:
Some of that may be personalized to me as a Swiss user of course. But it seems a bit much to be a coincidence. Maybe it is a loan word making its way from German into English now.
Some software uses license servers where each client is supposed to request a seat temporarily. If more people use it at once than seats were licenses they detect that with phone home features. We had that with Matlab I believe, if you tried using it in the most popular time you might not be able to.
In general media files can be formed in a way to trigger some bug in the media player, sometimes in ways that allow to overflow buffers and start ROP chaining.
About 8 years ago there was this media file going around crashing any iPhones that tried to play it with the integrated player.
Of course crashing is way easier than code execution. So overall your scenario is unlikely. VLC also does not yet know of any issues with 3.0.20: https://www.videolan.org/security/
And these days magnet links are everywhere, making it even a little simpler