I had a similar problem when my DS-5 instance was crashing frequently due to an Eclipse bug. When I restarted DS-5 I would receive the error you posted. A Google search for that error came up with this solution:
http://www.altera.com/support/kdb/solutions/fb145963.html Which was not helpful because I knew I didn't have an instance running. After some experimentation with "ps -ef" I discovered that my problem was the "vstrm_serverd_rddi" process still running in the background. I manually killed the process and I was immediately able to reconnect to my debug target. Try that and see if it helps.