I have a very similar problem. At first it was the install.php who could not run require_once of admin-functions.php (deep in the the required tree). Even using an include statement cause an HALT. I kind of pass that successfully by commenting it out (did not need it to install, just for upgrade).
But, when login in, I got that same problem with wp-login, that could not require_once wp-db.php (exactly as describe by trondert). Again, I got an HALT, but I can directly call wp-db.php with my browser, and it parses fine (except that it misses variables lower in the require tree).
Hope we will find a solution!
