Mavericks Server: Unable to use 3rd party SSL Certificates with Open Directory
I believe Apple fixed that bug in Server 4 but I can't reproduce it on Server 5. Have you checked the serveradmin command line tool to check your settings and status:

sudo serveradmin fullstatus dirserv
sudo serveradmin settings dirserv
sudo serveradmin -x settings certs


The -x argument instructs serveradmin to format the data as XML, which often shows more information (at least in the case of certs).

Sometimes you can set things there that the GUI balks at - but you also want to have the ability to call AppleCare for support or a great backup plan that's tested before mucking with sudo and serveradmin from my experience.