Thank you for your post!
You might start with these steps since it happened in FileMaker Server 14 as well:
1. Uninstalling FileMaker Server 15.0.2
2. Rename the FileMaker Server Folder to "FileMaker Server Old" -this will exclude any damaged installation files.
3. Update the Operating System to OS X 10.10.5 (Download OS X Yosemite 10.10.5 Combo Update)
4. Reboot the machine
5. Download fresh installer of FileMaker Server 15.0.2 - Reinstall
You could also run recovery on your Database Files before uploading them and setting Server back up for client use.
Also, are you using any Plug-ins?
If so, you might verify they are up to date and compatible with FileMaker Server 15.
Tank you for the answer, I will try that tonight.
We are not using any plugins.