When upgrading a phpBB 2.0.22 installation to 3.1.0-RC2, I receive the following error, which prevents the phpbb_users table from being converted:
Array ( [message] => Unknown column 'user_pass_convert' in 'field list' [code] => 1054 )

I believe the issue was caused by removal of that column in PHPBB3-12352, specifically commit https://github.com/phpbb/phpbb/commit/4698f6928e44a24a7a10ff8b4fed2c1a24cab338 but not removing the column from the converter (https://github.com/phpbb/phpbb/blob/develop-ascraeus/phpBB/install/convertors/convert_phpbb20.php#L900).