Okay, I encountered this problem twice in the last two weeks.
The first time, I did what no1tmorrow recommended, and it worked:
Delete the FileMaker Server Admin Console Java Applet.
Uninstall FileMaker Server, Java, and Bonjour.
Delete the Program File folders for FileMaker Server, Java, and Bonjour.
Disabled all of my Network Adapters.
Reinstalled FileMaker Server.
Enabled all of my Network Adapters.
However, I was unable to do this when installing FileMaker Server remotely. I was also able to fix the problem by following the instructions for resetting the FileMaker Admin Console Password (Search FMKB: 6989):
Delete the FileMaker Server Admin Console Java Applet.
Navigate to the FileMaker Admin Conf directory:
[drive]:Program FilesFileMakerFileMaker ServerAdminconf
Delete the following 4 xml files:
managerconfig.xml
server_config.xml
wpe_workerconfig.xml
ws_workerconfig.xml
Restart the FileMaker Admin Server from the command line:
[drive]:Program FilesFileMakerFileMaker ServerDatabase Serverfmsadmin.exe restart adminserver
This problem had been driving me crazy for days. I hope this helps someone else!