virtualbox.org

Did some googling, but can't find this specific error or a solution. If I missed something please let me know so I don't need to bother the forum in the future!

Good on you for trying to google! One way to get good results with a Virtualbox error is to find the UNDERSCORE_SPACED_CAPITALIZED words in the error message then put that phrase in a search bar with the text:

site:forums.virtualbox.org

added. The 'site:' function forces the search site to gather results only from the referenced website, works on any website that allows search engines in.

For guru meditations you need to search the log to find the UNDERSCORE_SPACED_CAPITALIZED error message. Search the log for the word 'meditation'. the next couple lines has the error.

In your case the error is INF_EM_TRIPLE_FAULT. Searching this reveals that your guest OS tried to do something so unusual it blew past all of the error-checking routines in Virtualbox and there was nothing to do but give up.

What the guest OS tried is probably related to this:

00:00:01.752652 OS Product: Windows 10
00:00:01.752653 OS Release: 10.0.17134
.........
00:00:01.867971 File system of 'C:\Users\Simon Dawkins\VirtualBox VMs\Capi\Snapshots' (snapshots) is unknown
00:00:01.867986 File system of 'C:\Users\Simon Dawkins\Downloads\OS X 10.11 El Capitan Retail by TechReviews\OS X 10.11 El Capitan Retail.vmdk' is ntfs

If in fact this is really Mac OS on a Windows host, then that's not a supported or legal setup. See OSX guests on non-Apple hardware.
The admins will be along shortly to lock this, and the Apple men in black will be around at your place soon.