Funny thing... I received a message from mignulikz in another forum :) The forum mentioned uses the same engine as Sumo Forum.
The messages originated from a well known Russian/German company. They have a spam-bot-net, which has been spamming forums with this kinds of stuff for at least six months.
There are certain measures that could be taken to make sure that this does not happen again, but I'm hesitant to go that far because so far these attacks have been one-offs and the solution would inconvenience legitimate users. And I just received word that Invision is working on a PM flood control system, which should mitigate the problem.