This has been driving me crazy for a couple of weeks now so I wanted to blog the solution and hopefully help someone else that runs into this.
A couple of weeks ago, I rebooted my machine, logged in and launched MS Outlook. From that point on, every time I created a new email, replied to an email, create a meeting request, open a meeting request or one of other numerous tasks, I would get a prompt saying: "Could not install the custom actions. The object could not be found"
I had no idea what that meant but assumed it had something to do with the Outlook install on my machine. I took a look at the setup logs and patches applied but couldn't figure out what was going on.
Finally, after some more searching online, it looks like one of my caches got corrupted. To fix my problem, I had to search for frmcache.dat on my machine. Make sure you enable searching for hidden files. Once you find it, delete the file (or rename it to be safe) and then relaunch outlook.
That solved my problem and no more prompts everytime I try to create a new email.