Sorry for the late response -- I was expecting an email notification about replies. Anyway, I updated my copy of another third-party add-on that I'm using (iGUI; it also uses iTween) and the problem disappeared. But one interesting thing to note is that simply updating iGUI (in my already existing project) thru the Asset Store did not fix the problem. I had to start a fresh project using the latest version before I could get iGUI, PlayMaker and iTween to all play nicely together.
[EDIT] Quick update: I have also determined that this problem can be caused by having an unnecessary copy of iTween.cs in your project. I added Playmaker to an existing project (which already had all latest versions of my add-ons) and the problem re-appeared, but this time only with PM calls to iTween! I located a folder with Plugins named iTween with iTween.cs in it. Deleted the iTween folder and now everything is fine. Apparently, my scripts are now calling iTween in one of my add-ons. Moral of the story: if you are calling iTween from your scripts, then you'll need iTween somewhere in your project, but multiple copies of iTween in your project may cause problems. If you run into a RetrieveArgs() error from iTween, look for an iTween folder in Plugins and delete it -- that should fix it. (But you may also need to update your add-ons to their latest versions.)