I figured it out by poking around in the browser code with "Developer Tools." Turns out, it was trying to read this code that saves and loads the screen dimensions in desktop mode, but couldn't access the files for some reason. But, this code is only relevant for the desktop version anyway, so I just commented it out.
//Load config
var configfullscreen = true;
var configheight = 1080;
var configwidth = 1920;
var configgamepad = 0;
if (file_exists("file_config.txt"))
{
var configfile = file_text_open_read("file_config.txt");
configheight = file_text_read_real(configfile);
file_text_readln(configfile); //Next line
configwidth = file_text_read_real(configfile);
file_text_readln(configfile); //Next line
configfullscreen = file_text_read_real(configfile);
file_text_readln(configfile); //Next line
configgamepad = file_text_read_real(configfile);
global.gamepadID = configgamepad;
window_set_size(configwidth, configheight);
window_set_fullscreen(configfullscreen);
file_text_close(configfile);
}