After reading again your first post looking very carefuly if I missed some info, this are my findings.
From your fist post:
My goal is to have a small windows 10 running in RAM loaded from USB. I have done this with XP and windows 8, but now I am stuck with windows 10.
Does the 8 VHD boots fine as filedisk and ramdisk on your current USB device?
If the answer is NO, your USB maybe is not working fine or is not fully compatable with your PC under certain circunstances.
If the answer is YES, then the problen could be related to the following:
steps I made:
removed the useless things from windows 10 with Winreducer.
install the produced ISO with mode: LZX into VHD with WinNTsetup 4.2.1
run the vhd in virtuablx to check anything running smooth and install svbus 1.1 while in virtualbox running.
If you installed the Virtual Box additional package it has its own drivers and other things to improve the VM funcionality, perhaps this overwrited the original USB drivers, part of them, or the services required to start the driver, the entries on the reg file controling the timings of this service, etc
About SvBus driver, on post No. 115 the author says:
SVBus V1.2 Virtual SCSI Host Adapter for GRUB4DOS
Today we present the new virtual SCSI driver SVBus version 1.2 for use with GRUB4DOS. SVBus V1.1 was not working correctly in combination with Windows 10 x64 Build 1803 and later. This could lead to a system crash with BSOD and the error CRITICAL_PROCESS_DIED. We discovered a Microsoft bug in esent.dll, which lead to these critical errors. Details are listed in the ReadMe.txt topic named "Application and Service Crashes on Windows 10 x64 Build 1909 with SVBus version 1.1".
In addition we added mode page caching for the MODE SENSE (6) command. The virtual HDD needs MODE PAGE CACHING to display the tab Policies in the Properties of the HDD in device manager.
To download the newest version use the link in my first post.
Much respect go out to all the heroes who work very hard in hospitals all over the world at the moment to fight COVID-19.
Keep up your amazing work and spirit!
Better use SVBus version 1.2 on a new build, and better don't use any Virtual Machine, you don't need it, create the VHD on a partition of your PC, WinNTSetup will create the entry on your PC BCD to let you boot from it.
I noticed in your last post you followed more or less the instructions from SVBus author, they are not bad but in my next post (to not miss the target of this post), I will suggest you my manual way, or the automated way using WIMBOOT_VHD by wimb.
For next comment I will consider the USB device is OK:
Abut the use of Winreducer: as the 10 OS on the VHD boots fine when loaded on RAM (by grub4dos) and only fails when starts file booting there is also the possibility the USB funtions are not working to its full capacity because some part of the driver, the services required to start the driver, the entries on the reg file controling the timings of this service, etc were affected during the reduction with Winreducer.