Pretty hard to suggest a fix based on such a limited description. Later versions of filemaker server have disconnect settings that disconnect the user automatically after the client has been idle for a specified time. I don't know if your version of Filemaker Server (5, 5.5, 6?) has that feature or not, but I'd check that out first.
Thank you for your post.
FileMaker Pro 8.5 and FileMaker Server 8.5 were the first versions to be certified under the Leopard operating system (10.5.x). You can either reinstall an older operating system, or try the suggestions from "PhilModJunk" (Thank you!). If the latter fails, then you may have to update to a newer version of FileMaker Server.
Is this after 30 minutes of inactivity or just always after 30 minutes? Does your computer go to sleep every 30 minutes? I noticed that once I changed my power settings, my filemaker does not shut down while my computer is still on. Make sure you check those settings - if the computer is scheduled to go to sleep, then you might want to check that first.