Designjet Z9+ port 8090 not reachable from Onyx Postershop

[ABK1]

I'm having an issue with my Designjet z9+ and Onyx Postershop.

Postershop can't communicate with the printer, it gives the error "*** HP DesignJet Z9dr 44: Alert: port(s) not accessible: 8090".

Printing from within W10 works without any issues (so I've installed a network printer with the latest postscript drivers). I can print directly from Adobe Acrobat to the network printer (bypassing Onyx Postershop).

My pc is connected directly to the printer via a Rj45 ethernet cable (so no switch or router between the pc and printer). 

Windows Firewall is turned off and no virusscanner is installed.   

My other printer, a Designjet z6800ps does work from within Postershop. 

I've contacted Onyx support but they said this is a printer-related problem. 

I can't figure out what port 8090 is used for. Since I have no issues connecting to the printer, I can also reach the internal webserver of the printer. So this works:
https://192.168.1.100/hp/device/index.html

This doesn't: 

https://192.168.1.100:8090

I've tried a second computer with a fresh install of Onyx Postershop 19.03 and the latest printer drivers. The result was the same;
Our Z6800ps can print from Onyx Postershop, the Z9+ not because of port 8090 is not accessible.
I'm really out of options. I'm sure it is no network-problem (since I'm connected directly to the printer and no firewall is activated). I'm sure the z9+ works fine (I can print from w10 and from an usb-stick). I'm sure the pc works fine (I did a factory reset of w10, installed ONLY Postershop 19.03 and the postscript drivers and disabled the firewall). 
Then as a last step, I did a factory reset from the z9+ and also reset the network card of the printer. I'm running the latest firmware: 06_19_41.2

Does anyone have any experience with this? 

[ABK1]

To answer my own question;

It turns out I had to change some settings on the Z9+.

Under Settings --> Printer settings I had to:

Turn Job Queue on

Set "When to start printing" to "After processing"

Nesting off.

The port 8090 seemed to be a sort of false-positive error since after changing the settings I mentioned above I was able to sent printjobs from Postershop to the Z9+ even though the error is still there.