Our network person and I have done a bunch of troubleshooting and came up with this summary and a few questions.

Ever since we installed FileMaker 16 in our two-machine setup, Zulu has been failing. In the Zulu error log from C:\Program Files\360Works\Applications\logs:

Oct 2, 2017 8:44:59 AM PDT com.prosc.mirror.config.server.FileMakerServerInfo getHostAddressForProperties
INFO: Will use 127.0.0.1:16020 for direct WPE address.

Oct 2, 2017 8:45:05 AM PDT com.prosc.fmpjdbc.FmXmlRequest doRequest
INFO: Starting request: http://127.0.0.1:16020/fmi/xml/FMPXMLRESULT.xml?-db=DatabaseName&-lay=ProscNoSuchTable&-view

Oct 2, 2017 8:45:05 AM PDT com.prosc.fmpjdbc.FmXmlRequest setProductVersion
WARNING: Null product version; assuming 12

Oct 2, 2017 8:45:05 AM PDT com.prosc.fmpjdbc.FileMakerException <init>
INFO: com.prosc.fmpjdbc.FileMakerException: Error 959: XML Web Publishing is not enabled - Run the FileMaker Server deployment assistant and make sure that the XML web publishing checkbox is selected () username: domain\user / requestUrl:http://127.0.0.1:16020/fmi/xml/FMPXMLRESULT.xml?-db=DatabaseName&-lay=ProscNoSuchTable&-view

 I verified that the URL does indeed return an error code of 959. Playing around with it, I found that if you use the DNS name of the machine instead of the localhost IP:port (https://wpe.domain.edu/fmi/xml/FMPXMLRESULT.xml?-db=DatabaseName&-lay=ProscNoSuchTable&-view), the request returns an error 105 (expected response) instead. 
 
I poked around on the internet trying to find out more about this, and discovered the -dbnames command; again, it works fine on the main https port (https://wpe.domain.edu/fmi/xml/FMPXMLRESULT.xml?-dbnames), but returns a 959 on the Custom Web Publishing port of 16020 (http://127.0.0.1:16020/fmi/xml/FMPXMLRESULT.xml?-dbnames). So it’s like XML is enabled on port 443, but not for the Custom Web Publishing port 16020. How is that possible?
 
For reasons we don’t fully understand and haven’t figure out how to correct, both the FileMaker Master and the Worker machines ended up with the Web Publishing Engine. So I tried the port 16020 request on the Master server, and it works! Which is great, except we want it to work on the Worker machine, not the Database server which ended up with a WPE.
 
Some questions:

1)      We know XML is enabled on the main (port 443) site, what do we need to do to make work on the localhost:16020 site?
a. I hope this involves moving the ‘Web Server’ listed in the Admin Console to the Worker machine, but if not any info on what might be going on there would be great.
2)      If we can’t get the Worker machine to realize XML is enabled on port 16020, can we change the FM config so Zulu doesn’t use port 16020 ‘for direct WPE address.’?
3)      If we can’t do that, are there any issues with installing it on the Master, other than the obvious issue that we don’t want the WPE on that machine?

Thanks for any insight anyone might have.

Did you ever get this resolved?  We are experiencing the same issue.

We solved it by moving Zulu to the primary machine in our 2-machine setup, rather than the web direct secondary machine.

We just had this issue recently after the client upgraded their FM server. Seems like XML API which we used to communicate with the CRM was switched off by default. Also, the guy who is their FM admin said that it's impossible to turn it back on via GUI so he had to turn it on via command line. Basically, XML functionality was not enabled.