Discover cannot access the list of new software to be added. I'm new to linux and I found the same problem in the 3 computers I've installed Kubutu. This bug is the same as 376241. I can see that there is a solution to this, but I can't install the newer version, cause I've have kubuntu 16.04 long term support. There isn't a newer version in the long term repository. Please advice how to install newer version or how to add newer version in the repository.
You can try adding the kubuntu-backports repository. Please contact kubuntu either way, there's not much I can do in this regard. *** This bug has been marked as a duplicate of bug 376241 ***