Windows to go installation unsuccessful, how to remedy this?

I have Windows 10 as 'Windows to go' installed on a Samsung T3 SSD external drive (500GB). Windows to go is a fully functional OS that I've been running on my MacBook Pro Retina (mid 2015). The only annoyance is that I have to switch between OS:s, shutting the OSX down to access Windows. For this reason I would love to start using Windows via Parallels in stead.

I followed these instructions, with no luck: https://www.parallels.com/blogs/windows-to-go/
The issue I have is exactly as described in this thread:
https://forum.parallels.com/threads/running-a-windows-to-go-usb-vhd.330249/
I get that exact same error message and Windows simply does not boot. I've used one full day yesterday and most of today to try out different tweaks to the settings one by one, to no avail. I even installed older versions of Parallels, up to version 11, to see if that would help, but no cigar. I'm currently on the Parallels desktop 13 trial (OSX 10.10.5).

This is extremely frustrating. I have some software installed on that Windows that isn't available anymore, and I need to keep access to it. Installing a new copy of Windows isn't an option, as I will lose the software. It's super cumbersome switching between systems and Parallels would be the perfect solution to this, if only I could make it work!