I think it's the same problem than you: https://bugs.launchpad.net/ubuntu/+source/update-manager/+bug/656602
Someone gave a trick to solve the problem and it's working on my computer.
I think it's the same problem than you: https:/ /bugs.launchpad .net/ubuntu/ +source/ update- manager/ +bug/656602
Someone gave a trick to solve the problem and it's working on my computer.