Make sure that  you use LDAP protocol version 3 in  your LDAP entry. You
can also try to set TLS_REQCERT to never in your /etc/ldap/ldap.conf.

The TLS support is very new in roundcube:
 http://trac.roundcube.net/ticket/1485104

I suppose nobody really tested it.
-- 
BOFH excuse #24:
network packets travelling uphill (use a carrier pigeon)