Yes, the simulator seems to be down for me as well right now.
Actually, I've noticed over the past week or so the sandbox as a whole has had something going on with it. The majority of my sandbox accounts no longer allow me to login (invalid credentials.) Some of them let me after I change the password on the account and some of them still don't.
The developer account was actually giving messages about how the sandbox accounts were unavailable at one point, too, and it wouldn't list any of them at all when that was happening.
So it's a bummer when the simulator isn't working well, but if you're able to process sandbox transactions and it's working there then that's a better test anyway.
Hopefully the issues will be worked out before long.
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