You're right. That's probably because of itch. I'll simply upload it to my website and link it from the game if someone has the same problem
Sanoke (sanoke.itch.io) figured out a way to make it work. Here's a commit showing what to change (on my game, which had the same issue): https://github.com/thepaperpilot/SolarTD/commit/41b7704f9c4bd215128a9fe74c7aeff8e71ab7c9