I have just found out that you actually are banned from their website if you still get the "down for maintenance" error.

I have got this issue for almost 4 weeks now. Wrote 2 emails to the admin of csgodicegame and asked here for an explaination of the issue without ANY reply.

I tried several tips from other users, like changing the browser or deleting the cache and cookies from my browsers. It didnt work anywhere. Then, I tried to log in with my alternate account and it worked perfectly fine.
This as a fact, the only reason I can imagine for the error (down for maintenance) still appearing is that I am actually banned. My coins and skins are stuck and I dont think I will ever see them again. With other words - they just scammed me.