I reinstalled the script and it still doesn't work. I'm using 2.0.21. When you click on "registration" it comes up with a blank page.
I don't have any other mods applied to phpbb, and the mod is so simple that I'm doubtful that I made an error.
I have GD extension enabled.
Could it be affected by file permissions? (This is a total guess, eg an inability to read/write/execute the file?).
Do any other php.ini settings matter?
Note: I got this to work fine for me on another site running 2.0.20