I am looking for a solution to:

An author publishes a post on the WordPress site (not on a WordPress plugin based forum). Another registered author wants to comment the post, but not in public. There has to be a function in the post that makes access to the following:

1. Write a private comment attached to the post that is only visible to the two authors (the admin might also be included)

Or

2. Write a private massage to the author of the post by clicking on an icon or something like that, which create a text field and the author of the post is automatically the receiver.

I haven’t managed to get the following plugins to do it the way I have described.

BP Better Messages, BuddyPress, Front end PM, Komentbox, Ultimate Member, WP Comments Fields Manager, WP Grade Comments, WP Private Comment Notes

regards,
Mikkel

I noticed that if these plugins crash or are disabled, then all the private comments will become public. That’s not a good thing. It could compromise security and create a high number of very unsatisfied users.
regards,
Mikkel