Hey leafo! Thanks for your quick response! Yeah, I figured it out... I hope this lesson helps someone in the future, this is what happened:
I created the game with Game Maker, so the exported game includes an index.html and a folder containing its assets. It just so happens that I created a custom html to be my index file and set it as an included file. When GM exported the game, it was also exporting my custom html file inside the assets folder, so I had 2 index.html files: one at the root folder of my game and one in the assets folder. For some reason, itch was loading the one in the assets folder instead of the one at the root, so yeah, it obviously wasn’t able to find its assets in a relative path to the assets folder. Finding that the embedded game wasn’t changing te index.html source code no matter what I changed in it was key to figure this out... I deleted the unwanted index and the game is back in business.