How can I configure iPXE menu settings to boot Plop Boot Manager?

I have created a big iPXE server based on an Ubuntu Server 22.04.1 system. I have entries for different Linux images, Windows installers and Clonezilla Live.
Now I want to include also an entry for Plop Boot Manager, and I’m finding that there is not any documentation available to do it, even in the plop boot manager official site.

I have a boot menu called boot.ipxe, configured based on this guide.

I have tried to add a simple entry to boot plop boot manager, but when I try to start it, it fails. This is the content of the entry:

:plpbt-5.0.15
kernel http://192.168.1.2/os-images/plpbt-5.0.15/plpbt.bin
initrd http://192.168.1.2/os-images/plpbt-5.0.15/plpbt.img vga=1 nfsmount=192.168.1.2:/free/pxe/os-images/plpbt-5.0.15/ nomodeset
boot

The resulting error, when I try to boot that entry, is as follows:

http://192.168.1.2/os-images/plpbt-5.0.15/plpbt.bin... ok
http://192.168.1.2/os-images/plpbt-5.0.15/plpbt.img... ok
Could not boot: Error 0x2a818006 (https://ipxe.org/2a818006)
Could not boot: Error 0x2a818006 (https://ipxe.org/2a818006)
Insert system disk in drive.
Press any key when ready....

The root of the http server (apache2) is well configured (I use it already for other entries), but just in case you need it: it’s /free/pxe
The NFS server is configured to share /free/pxe (read-only)
I have tried to search for some documentation on what kinds of files does the iPXE menu need to recieve as kernel and initrd, but I think there are not a lot of information on this matter.

So does anybody know how should I configure the iPXE entry for Plop Boot Manager?

Let me show you the tree command for the plpbt-5.0.15 directory:

.
├── 1README.TXT
├── doc
│   ├── liesmich.html
│   ├── liesmich.txt
│   ├── readme.html
│   └── readme.txt
├── experimental
│   └── usb4ksector
│       ├── 1README.TXT
│       ├── plpbt.bin
│       ├── plpbtrom.bin
│       ├── plpinstc.com
│       └── plpinst.com
├── install
│   ├── plpbtin.img
│   ├── plpbtin.iso
│   ├── plpinstc.com
│   └── plpinst.com
├── licence.txt
├── Linux
│   ├── 1README.TXT
│   ├── plpbt.bin
│   ├── plpbtrom
│   ├── plpbtrom.bin
│   ├── plpcfgbt
│   └── plpmkboot
├── pcmcia
│   ├── install
│   │   ├── plpbtin.img
│   │   ├── plpbtin.iso
│   │   ├── plpinstc.com
│   │   └── plpinst.com
│   ├── plpbt.bin
│   ├── plpbt.img
│   ├── plpbt.iso
│   └── plpbtrom.bin
├── plpbt.bin
├── plpbt.img
├── plpbt.iso
├── plpbtrom.bin
└── Windows
    ├── 1README.html
    ├── InstallToBootMenu.bat
    ├── InstallToMBR.bat
    ├── plpbt4win.exe
    ├── plpbt.bin
    ├── plpbtrom.bin
    ├── plpbtrom.exe
    ├── plpcfgbt.exe
    ├── plpcfgbtGUI.exe
    ├── plpinstc.bin
    └── plpmkboot.exe