Exception logging is disabled by default in Community Server 2007. To enable it simply edit the communityserver.config in the root of your web site in a plain text editor, for example Notepad.
Locate the line in the core section near the start of the file:
enableExceptionLogging = "false"
and change it to:
enableExceptionLogging = "true"
Save the file. In order for the change to take effect it will be necessary to touch the web.config
Product: Community Server 2007