It does say in the release notes to do a clean install, not an upgrade.
I installed LE 10 on fresh SSD card, after approximately two weeks exactly the same happened. First I got info during boot that the network connection failed but I was still possible to boot after long time. Now I can't do anything, I have just bootloader error. I've come across with exactly the same issue when I had been testing the nightly builds.
Thank you.
It does not work with any method.
Upgrade does not work and making a SD-card from scratch also not.
With the upgrade method at least, I saw a picture for a short moment. With a SD-Card new - nothing 
