Thank you! This eliminates a number of possibilities, such as some sort of issue with full disk encryption, or a "jump" upgrade. You are attempting to upgrade a -current system from July 22.
There were two changes to the
install.sub script since July 22, but neither seem to apply to this. And there was a single change to the amd64-specific
install.md on July 28, which also doesn't appear to apply.
Based on the information you provided, I believe your best bet is to submit a problem report to the bugs@ mailing list. Here are two links that can help:
http://www.openbsd.org/report.html
http://www.openbsd.org/mail.html