The initial issue that caused the resurrection of this thread on 23/05 has been resolved. However, we are still seeing the occasional, smaller dip in population across the EU server. Our network engineers are in the process of investigating.
However, there is also a chance that some of the issues mentioned here in this thread are not actually caused by these server hiccups but the client's connection itself. So, just in case your particular issue can be eliminated, we highly recommend you read through the Diablo III Connection Troubleshooting support article as it may help you to get back in the game.