I strongly recommend that you remove permissions for fileMaker clients to be able to write to the folder that contains the database files being hosted by server. Only the server needes write permission to this folder.
Here is one possible issue you can investigate:
For More Information see: Can't login to FMS database on new Mac, other Macs can
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip