I tried to reproduce it, but it works in my case. I did experience the white prefab issue up until I added the GSPAWN_HDRP macro. Then I refreshed the previews and it went away.
Prefab previews do appear darker in HDRP though, I don’t know why that is and they seem to change color when you interact with them.
Please make sure you are using the HDRP version of the Lordenfel package (or any other package you might be using).
First of all, thank you very much for your help. Unfortunately, the problem still persists for me. I also noticed that when I look at a standard HDRP/Lit material, in the lower right corner of the Unity Editor where the material preview appears, it also shows as white without any texture. It might be related to the graphics card or possibly a Unity bug. If I find out anything more, I’ll get back to you.
I found the solution in my case. There are two different HDRP templates: High Definition 3D Core and High Definition 3D Sample. I used the Core template.
Follow the instructions in the documentation at Assets\GSpawn - Level Designer (PRO)\Docs\GSpawn - HDRP Integration.pdf, but additionally perform the following step as 2.1 if you are using Unity’s High Definition 3D Core Template.
Go to Project Settings → Quality to identify the current quality level or, in the following step, adjust all quality level assets. Then, in Project Settings → Quality → HDRP, select the currently used quality level or, for all levels, set Lighting → Light Probe Lighting → Light Probe System to Light Probe Groups.