The tool database for the mix'n'match tool, p50380g50851__mixnmatch_p, is on the wikidatatwiki_p server. It seems to have vanished.
i.e. on labsdb1002, which was down for maintenance. In preparation for that, wikis were redistributed over the other database servers, as was announced on labs-l: http://thread.gmane.org/gmane.org.wikimedia.labs/3773
I think the correct solution is to explicitly state the database server if you need a user database, instead of using wikidatawiki_p.labsdb; in this case, that server would be labsdb1002.eqiad.wmnet. CC'ing yuvi and coren to chime in.
Hmm, so yes - you could have used c2.labsdb to connect to it, but that's not really documented anywhere (afaik). Connecting to labsdb1002 now will also be not very helpful, since the wikidata copy there is still resyncing.
We should document things better!