My apologies, you were absolutely right.
The game had a critical bug on some browsers and didn't work at all, luckily I had the same issue at my work computer.
I found a way to fix it, it should work now. :)
If you're interested in the details, the problem was that my maps were PNGs, and it seems some browser versions change the colors slightly, which made the maps undecipherable. I changed the files to BMPs and it's working fine now. (and is even a smaller file size)