My copy of Seedcode Calendar (build 5.4) is getting very slugglish and temperament and often crashing. I discovered that even when there is only one client connected to the server, FMS indicates that there are always between 20 and 40 clients connected, all of them via 127.0.0.1. Could this be the source of the problem? And if so how to fix it. Or is there any other maintenance that can improve the situation?

The first thing I would do is restart the server to try to clear out those phantom clients. Moving forward, make sure your users aren't in the habit of force quitting FileMaker while connected to the server, and that no one is losing their connection because of a problem with the network. I've seen phantom client connections persist when FM Server isn't properly notified of a disconnection. If it continues to be a problem you might try searching FileMaker's knowledge base: http://help.filemaker.com/ or forums: http://www.filemaker.com/support/forum_selection.html for other FileMaker users who have seen similar behavior. I hope that helps!