Off the top of my head: try VPN, try cookie method of connection, completely flush your browser (not just cache), change your system time to JST, reset your router.
I’ve tried doing all of the above but can’t seem to connect to the game. As it stands, only mobile version works.
Update: I’ve tried using different browsers, VPNs and other methods and still doesn’t work. Besides that, I’ve also tried playing other japan-only dmm games and got a different error instead:
このゲームはお住まいの地域からはご利用いただけません
現在、お住まいの地域からご利用いただけるゲームはございません。
This also appears for Kancolle for different browsers