I had the same issue and fixed it myself or at least it seems fixed. Did you happen to upgrade this environment or was it a clean install? What I found was there were duplicate records in the network_logical table. I think this is due to faulty upgrade scripts. Since this was one of the major changes in 5.1 update it makes sense in a way that this would be where the bugs are introduced. If you look in that table I will bet you will see some duplicates for the network in question. I removed the duplicates and was then able to update/remove the network.
Paul