The error message you are receiving is indeed a result of a bug in the extesnion manager. see the following post on the Adobe forums:
I can see where you may question this when installing other extensions does not produce the same error, there are possible reasons for this though. Those other extensions may not need to update the Dreamweaver menus. If those extensions where already installed, the access points may already exist so there is no need to update the menu. I cant say for certain why the other extensions will install, but those are a couple of possibilities.
nobody likes to have to reinstall their extensions, I can understand that, but from time to time, the extesnion folder may become corrupted.
Looking at the official Dreamweaver troubleshooting guide, renaming the config folder is step 10 of 21: