Guess SR didn't got that one too; that's why they closed registration.
The 502 errors aren't server-load related, but Tor related. They depend on the "road" you took and the nodes along it. If they're fast it will look like you're in the plain open internet, if slow... well... they'll timeout.
Now, you can try to ask Vidalia to look for a new road by pressing "Change Identity" and hope to get one less bumpy than the one you're already.

BlackMarket is receiving more than 1 new registration per minute without any issue.