Thank you for your post.
I'm assuming the log file you are referring is the access.log file? Is there any other information displayed for that user in that file?
Since this used to work, something has obviously changed. If nothing comes to mind, then try rebooting the servers (FileMaker Server and server where external authentication information is stored). If that doesn't seem to work, then uninstall and reinstall FileMaker Server.
Please keep me posted with your progress.
Thank you for the reply. After contacting Preferred Support, they offered the same solutions. Unfortunately, I contacted our Windows administrators and it turns out that the failures in authentication can most likely be linked to the upgrading of the Active Directory structure to 2008. It seems that OS 10.5 on the Mac is no longer properly communicating to the Active Directory domain for credentials, which in-turn causes the issues with the external accounts in Filemaker.
Since the workstation/FMS system.log didn't show any errors, it was difficult to determine if the issue was with the external servers or the Filemaker server.