All over the internet I've seen reports of APFS conversion issues:
- An internal error occurred while preflighting your volume for APFS
- APFS converter preflight returned 6
- APFS converter exit code is 1
- Exiting conversion operations with exit code -69860
But no one has solved the issue without the nuclear option of a fresh install.
Really simple fix: Install Paragon Hard Disk Manager trial, and shrink your system HFS+ partition by a few MB (I shrank by 100MB). The problem is instantly resolved, don't even need to reboot.
Whatever weird state my partition had got into was solved by this resizing operation. Disk utility first aid was always clean, so not sure what went wrong.
This was on a 2013 MacBook Pro Retina with bootcamp installed