My b2evolution Version: 1.9.x

Just upgraded from 0.9 to 1.9 - a Fantastico-based upgrade. I've managed to get my blog itself accessible again, courtesy of the wiki, so that's good, but I can't log in to the backend.

I've searched, but can't find the error message I get when I try:

Notice: Undefined variable: admin_dirout in /home/oneandon/public_html/geekblog/admin/_header.php on line 17

Notice: Undefined variable: core_subdir in /home/oneandon/public_html/geekblog/admin/_header.php on line 17

Warning: main(/home/oneandon/public_html/geekblog/admin///_main.php) [function.main]: failed to open stream: No such file or directory in /home/oneandon/public_html/geekblog/admin/_header.php on line 17

Fatal error: main() [function.require]: Failed opening required '/home/oneandon/public_html/geekblog/admin///_main.php' (include_path='.:/usr/lib/php:/usr/local/lib/php') in /home/oneandon/public_html/geekblog/admin/_header.php on line 17

Any suggestions?