I'm editing this post because now, after about the 10th install attempt, I think I am finally understanding this.
When the install completes, I am presented with the login page. The DE is set to "Default." After entering my password, the same screen is presented again. But if I switch DE to "MATE," then I can log in.
Please ignore my previous post about this.
(Note, however, that the message about the locking still shows.)
Another error, just before the login screen, was "File based locking initialization failed."