Comment 9 for bug 1630327

I commented the "user-setup/encrypt-home" setting (comment #6) in the preseed file I used back then, and the user creation step was successful, but I didn't try to uncomment it again
.....
# Set to true if you want to encrypt the first user's home directory.
#d-i user-setup/encrypt-home boolean true
.....
after that I could install hundreds of xenial laptops quite smoothly (last ones yesterday)

As I try to guess in comment #6 above, setting this to true might have had side effects with the OEM user (required by oem/ubiquity)
I will try uncommenting that preseed setting again with last release of xenial today.