Before closing this issue, I was checking this thread due to a similar problem. I verified that the currentsite.txt had the correct site specified. When that was verified, and the problem persisted, I double-checked the erpnext.vm/site_config.json file. Indeed, I had a typo and not added double quotes to the line that enables developer mode:
Incorrect
"developer_mode: 1
Correct
"developer_mode": 1
Once I did this, the developer VirtualBox machine worked.