Recently i run into this issue with Windows Server 2008 (x64) & Outlook 2007 SP2 (online – which had earlier Cached mode, later changed back to Online) machine. Whenever i tried to open a message or create a new one, i get this error message - “Could not install the custom actions. The object could not be found”. Later i tried to have a look(bugging) into my machine. I found it’s because of the corrupt forms cache.
I fixed the issue like this.
1. Close Outlook.
2. Under C:\Users\ find the folder for the user who's experiencing the problem.
Note: Make sure under the Tools, Folder Options menu that system files are not hidden.
3. Go to \AppData\Local\Microsoft\FORMS, and delete the file FRMCACHE.DAT
4. After clearing the FRMCACHE.DAT, restart Outlook.
This steps resolved the issue!! Happy Outlook troubleshooting!!