M4301fdw "Guest: Print Failed" error when printing from database application

[Technovations]

I have a brand new HP M4301fdw printer connected via Ethernet (with IPv4) in a commercial environment. It is in a location that's connected to three other locations via site-to-site VPN. Each location has its own NAT'd private LAN IP address range (e.g. 192.168.1.xxx, 192.168.2.xxx, etc.). This particular printer is 192.168.4.131. It prints just fine from Windows applications - whether on computers in the same subnet or even from computers at other VPN sites. However, it will not print from our primary database application (InterSystems IRIS which is running on a high-end Windows 11 desktop PC in the 192.168.2.xxx subnet). Whenever we send a print job to it from that IRIS database front-end, we get a "Guest: Print Failed" error in the printer's job queue.

The database application does require us to use a universal PCL6 driver.  I have tried every iteration including all the older HP UPDs from version 5 up to version 7.2 and the newer HP Smart Universal Printing driver (which is actually what you get when you download the basic drivers for the M4301 from hp.com). None of them have allowed the printer to work from our database application.

Here's where it gets really weird: I installed two other brand new HP M4301fdw printers for the same commercial client on the same day and they both work just fine (even from the database app). The only difference is that they are located in the 192.168.1.xxx subnet. And, even weirder . . . the printer I'm having the trouble with (192.168.4.131) did work from the database app on the first day it was installed. it stopped working on the second day and has not worked since.

I opened a support case with HP on April 20th and it's still open. They have largely been useless. They had me try upgrading the firmware. They had me perform a factory reset. This week, they even replaced the printer with another brand new one just like it - the new one is exhibiting the exact same symptoms I described above. I'm pulling my hair out!!!

I explained to them that this seems like a simple ACL issue but no one I've spoken to has been willing to assist me in trying to figure it out. I've searched online and have not been able to find much. I've gone through the "Security" menu in the printer's EWS and there are "access control settings" but they are not intuitive and I haven't been able to figure out if changing any of them would help.

Has anyone ever encountered anything like this? Do any of you know what access control settings I might be able to adjust in the printer's EWS to get it to just print every job without using an embedded "Guest" account?

Any assistance would be immensely appreciated.  Thanks!