I gave up after being stuck at Preparing... (1/2) for 10 minutes and reloading the page several times
Preparing phase ends in like a second really. Here is what you can do:
1) load the game in incognito mode. (it might be that the essential files to run the game didn't load correctly and the corrupt version is cached)
2) If that doesn't solve the problem, open the Console. Right click anywhere on the page --> Insepct --> Console tab --> reload the page and show me whatever error you get.