Cannot set proxy for Kmail

[giacof]

I'm using Kmail on Kubuntu 15.04:

uname -r
3.19.0-18-generic

apt-cache policy kdepim
kdepim:
  Installed: 4:4.14.6-0ubuntu1
  Candidate: 4:4.14.6-0ubuntu1
  Version table:
 *** 4:4.14.6-0ubuntu1 0
        500 http://it.archive.ubuntu.com/ubuntu/ vivid/universe amd64 Packages
        100 /var/lib/dpkg/status

I tried to set a proxy for Kmail from the menu Settings -> Configure Kmail... and then selecting the Proxy tab.
I expected to find a form to enter proxy settings, but instead I read the following errors:

Cannot find module

The diagnosis is:
The desktop file was not found

(Note: text translated from local language into English)