Can't patch new engine client

I’ve just downloaded the “new” client to test the new engine, but the patcher does not want to start his job. I always get “The Anarchy Online client is already running. Click OK to try to close it.”. I click OK, nothing happens, and 10 seconds later another message appears “Could not close existing AO client. Please close it manually and click OK to continue.”, and these two message boxes loop every ten seconds. Nothing else happens.

There is no other AO client running. Starting the patcher as first thing after a fresh reboot does not change anything. Anyone got a hint what is going wrong here?