According to the BBC:
"We are praying that the Lord of history purifies our memories of every prejudice and resentment and allows us to freely proceed on the road of unity," he said. He referred to the "painful episodes of history" which had cast a shadow over their relationship. "In particular, we cannot forget what happened in the month of April 1204," he said. "How can we not share, at a distance of eight centuries, the anger and the pain."
The “relationship” he is talking about is that between the Catholic and Orthodox Christian churches. There has been tension and conflict between the two for centuries. Recently matters have gotten somewhat worse as Orthodox leaders have accused Catholics of moving into Orthodox “territory” and trying to convert Orthodox Christians to Catholicism. Because of that, Russian Orthodox Patriarch Alexiy II won’t approve of a visit to Russia by Pope John Paul II — one of the few countries he hasn’t visited and would really like to travel to before dying.