I am a big fan of "Dave's Garage" and a long-term user of PVE so hope I can give a little something useful back to you in return.
The easiest way to rule out an issue with the Windows 11 would be to boot the Proxmox installer ISO inside a VM on your existing Proxmox install; select...
I would suggest looking at https://sourceforge.net/p/e1000/bugs/399/ - this sounds like something that I hit several years ago and specifying the module parameter detailed in the report may fix the problem you are seeing.
A very quick feature request for the Proxmox devs.
I am trying to get Proxmox 3.x working on some Apple Xserve (late 2006) servers which meet all of the Proxmox hardware requirements but they do not support booting via BIOS or 64-bit EFI.
This particular model of Xserve only...