Could just be a coincidence, but I had the same thing happen today, and I have been using jingle (voip direct from pc to pc from spark) for a week now, but today I let my client update from the internet to 2.5.1 and logged into the server, and I didn’'t have the dial (computer to computer) option when I right clicked on a user in my roster, but after I went into my preferences, made some UNRELATED changes, then went back to the spark window, lo and behold the DIAL option was there!! might help you to try that… just maybe!!
Try adding someone that is currently connected to the server to your contacs list, let them just go into the default friends group that it wants to create… when the group is displayed with them in it showing online, send them a message, go back into your roster and right click on their nake, you should then see a dial icon appear and when you go to that it should say computer to computer…Also is your server public or internal network only??