I am experiencing the exact same problem as detailed in ticket 1484703:
http://trac.roundcube.net/ticket/1484703No matter what is typed, when Spell Check is clicked, it returns the popup "No Misspellings Found." No errors are logged.
I have OpenSSL on the server, PHP is configured and works with OpenSSL and cURL. RoundCube is configured correctly, exactly as detailed in the ticket above.
Ticket 1484703 incorrectly claims it is a duplicate of 1484196
http://trac.roundcube.net/ticket/1484196 which is also spell-check related, but an entirely different issue.
I have spent hours fiddling trying to figure out what the problem is, to no avail. Has anyone else ever had this problem, or know what I might look at next to get around it?