Thank you for your post.
These errors, although appearing lots of times, should not affect FileMaker Server. This was introduced in Mac OS X 10.11 (El Capitan). I have found at least one discussion on the Apple forum about this:
Our Development and Testing departments are aware of this issue, and they are working with Apple to find a resolution.
I have this error with about every new FMS 14 installation I've done so far on El Capitan.
Finally found something that keeps the system log from flooding.
I'm not really sure if it's the right thing to do, but at least I don't see the flooding anymore, and FileMaker Server acts like it did before.
Step 1. Enable root.
I do this by going into Directory Utility via de users & groups control panel->Login Options->(authenticate)->Edit Network Account Server->Open Directory Uility. I can never remember the path of that app.
Once the Utility has started authenticate yet again and you'll have a "Enable Root User" somewhere in the "Edit" Menu.
Give it some password, you'll need this afterwards so try to remember it.
Step 2. Start terminal
Step 3. Stop FIleMaker Server completely
I do this using "sudo launchctl stop com.filemaker.fms"
We want to stop the helper daemon as well, so it has to be done this way.
Wait for all fmserver processes to stop, you can monitor it using the Activity monitor. Use "All Processes" and search for "fmserver" in the window toolbar. There are sometimes some OSX "housekeeping" processes that continue to run under the fmserver account after you stop fmserver_helperd, don't worry about them and keep them running.
Step 4. Change to the fmserver account by doing this in terminal:
mbpro:~ peter$ su
The su command changes you to root, use the password you've entered earlier for root
sh-3.2# su fmserver
From root we can su to fmserver without giving a password
If you did everything right you now have a terminal session open as the user fmserver.
bash-3.2$ cd /Library/FileMaker\ Server/Database\ Server/bin
bash-3.2$ ./fmserver_helperd -d
Change to the bin directory of FileMaker Server and start fmserver_helperd.
The console XPC errors should not appear anymore.
The terminal keeps displaying some output that I think would normally go to /Library/FileMaker Server/Logs/stdout and /Library/FileMaker Server/Logs/stderr but I do not care for that much. Maybe a good idea to keep it open and minimize the window.
I don't know why this works, but it works. Maybe it gives some insight on where the XPC error problem originates, it appears to be an El Capitan issue for which Apple's fix is long overdue now.
This is probably better then just starting it up:
./fmserver_helperd -d >> /Library/FileMaker\ Server/Logs/stdout 2>> /Library/FileMaker\ Server/Logs/stderr &
It redirects the output to the standard files and the ampersand in the end liberates the terminal.