probable there is wrong link. go to /usr/share/applications. you can see two synaptic applications (Synaptic Package Managers). with ls -l command you can identify real names. there are synaptic.desktop and synaptic-kde.desktop. the second one is not needed. desktop type files are as a matter of fact specific text files. you can read them with leafpad. check the line with Exec. it's simply improper program version (kde).
I'm using lxde and deleted second file. I do not know if your menu is automatically updated after deleting the file. if not you should probable edit ~/.config/openbox/menu.xml (sorry for 'probable' but I haven't use openbox wm for half a year or more).
cheers,
