What version of FileMaker Server 10 are you running?
Software Update: FileMaker Server and FileMaker Server Advanced 10.0.2.205 release notes
1.2. An authentication problem with FileMaker Pro clients connecting to a database hosted on FileMaker Server 10 running on Windows has been fixed. The authentication problem occurs when external accounts are used to log in to the server to view their list of databases. The server setting "File Display Filter" must be set to "List only the databases each user is authorized to access".