I have a .xsessions-errors message saying that the panel cannot find the evolution.desktop file. Indeed only evolution-2.4.desktop is installed.
evolution is not supposed to install in evolution.desktop. Some time ago, they switched to versioned .desktop files. You must have a reference to the old desktop file (before it was versioned) somewhere, such as on your desktop. Maybe the actual error from your .xsession-errors would help you track it down.