I had the same issue, but I didn't have any legacy CB plugins installed. I did try to install a couple of other CB plugins, and got that message when I tried to access them. Worse yet, on the front end of the site, when logged in, it would give me the same message without letting me into the site.
I disabled the plugins until I read this post. But since I didn't have any legacy CB plugins installed, my solution was different.
I went to the Joomla Plugin Management and enabled "System - Legacy" plugin.
I'm not sure what that had to do with the new CB plugins, but they work like a charm now.