As others have already noted, you're thinking of the function
dired-jump, which is provided by Dired-x (not Dired+), and is bound to
C-x C-j when the Dired-x library is loaded. (You can turn off this binding by setting
nil before Dired-x is loaded.)
C-x C-j does something, rather than complaining that
C-x C-j is undefined, what's happening is that something else has defined a binding for it. Whoever defined the binding last wins.
So you need to track down what other package that you're loading is overriding
C-x C-j. That package is loaded after Dired-x, so if you're loading
dired-x in your init file, it's either something that you're loading further on or something that gets autoloaded at some point.
C-x C-j doesn't do anything on its own, but waits for another key to be pressed, it's a prefix key. Press
C-x C-j C-h to see a list of bindings that start with this prefix. This should give you a good idea of the culprit package.
Malabarba's hypothesis that it's Jabber mode is a plausible one. Once you've identified a key sequence that starts with the prefix
C-x C-j, press
C-h k followed by that key sequence. This will show the help text for that command and the package that provides the command. It's likely that the package that provides the command is the same package that defines the binding.
It's unlikely that you'll need to go that far, but just in case, if there's some key binding you can't track down, see How can I find out in which keymap a key is bound?
If you want to load that package that overrides
C-x C-j, but you don't want it to take over that binding, see if it has an option to skip that binding. If it doesn't, you can snatch the binding back: arrange to run
(global-set-key "\C-x\C-j" 'dired-jump) after the offending package is loaded. It may be enough to put that at the end of your init file. If not, you can use
(eval-after-load "NAME-OF-THE-OFFENDING-PACKAGE" '(global-set-key "\C-x\C-j" 'dired-jump))