From here

http://area51.phpbb.com/phpBB/viewtopic.php?f=99&t=47576&p=277311#p277311

when using Google OAuth getting the following error.

General Error
SQL ERROR [ mysqli ]

Column 'user_id' cannot be null [1048]

Not sure if it is an issue with area51 or with the OAuth plugin for Google within core phpBB.