I was able to solve this issue by manually copying the FileMaker Advanced 15 folder from the /Applications folder of a working client to the this client. I also had to copy over the ~/User/Preferences/com.filemaker.client.advanced12.plist and ~/User/Application Support/FileMaker folder manually. Once these things were manually moved over, I was able to connect.
Not running in the Applications space in the OS is probably not recommended and may lead to other problems. Just be aware of that. Another issue that comes up is that there have been SSL patches since 13 and if the database has SSL turned on, 15 may have problems IF the database is hosted on a FM 13 server. What server version is it on?
For my 2 cents, I would say reinstall FMP 15.
FMA 15 is running from within the Applications folder, I simply uninstalled it from the malfunctioning client, and copied it over (via network) from a functional client rather then using the installer in order to fix the issue.
Server has been running on FM 15 for almost a year now
OK, my misunderstanding and glad to see the copy worked. Also good to see you on FileMaker Server 15. Keeping current sure helps in today's world of security! Good luck with it all!