Thanks for the prompt response.
Joomla user registration was turned off when I installed CB. I verified that it is still off (Joomla User Manager -> Options -> Allow User Registration = No). The default Joomla login module is also disabled (unpublished).
I checked the CB Core Database using the CB Tools option. One error was found and corrected.
I recreated the CB Workflows module, which I named "Moderator Actions".
With this done, the Moderation Actions (CB Workflows) module once again appears in the Joomla Module Manager, and on the front end (for users in the Special access group, which is as it should be according to the configuration).
Unfortunately, the module still shows No Pending Actions on the front end, even when there is a user waiting for approval. (Moderator e-mails, however, are being sent without any problem.)
As I noted previously, I've configured CB on several other sites without seeing anything like this issue, so I'm not sure what else to check.