Seems to be a bit of a conflict with Codepress Admin Columns, just so's you know.
I'm assuming it has to do with unsetting and resetting the Posts column near line 310 in co-authors-plus.php. With both plugins activated and the 'Posts' column checked within CP Admin Columns, you'll see two 'Posts' columns on the user list table. One empty column (Co-Authors Plus) and one populated column (CP Admin Columns). Each column's visibility can each be controlled via screen options but both have the same name so it might confusing for other people.