Thank you for your post.
Before proceeding further, I want to make sure you first read Knowledge Base Article #5508:
... and the accompanying PDF files. This covers all the basic questions I would ask first.
Are all the machines up to date in patches for the OS and FileMaker Server and the OSX Server WorkGroups?
In the FileMaker Server properties for the security tab, is FileMaker and External Server accounts selected?
In the database file, make sure there are defined account privileges that are assigned to External Server Accounts that match the group. Check the group permissions and check the authentication order.
Keep me updated with any progress.