When you have a MOD installed for 3.0.12 that adds a custom CAPTCHA method (such as this one https://www.phpbb.com/customise/db/mod/sortables_captcha_plugin/ ), you run into problems when upgrading to 3.1.0 b2.
The update itself doesn't throw any errors, but the DB entry for captcha_plugin isn't altered. Since there's a big chance that the equivalent 3.1 extension version is not available directly, this entry will not be correct.
When accessing the "spam countermeasures" page, the result will be this:
[phpBB Debug] PHP Warning: in file [ROOT]/includes/captcha/captcha_factory.php on line 35: include(./../includes/captcha/plugins/phpbb_captcha_sortables_plugin.php): failed to open stream: No such file or directory
[phpBB Debug] PHP Warning: in file [ROOT]/includes/captcha/captcha_factory.php on line 35: include(): Failed opening './../includes/captcha/plugins/phpbb_captcha_sortables_plugin.php' for inclusion (include_path='.;C:/php/pear')
[phpBB Debug] PHP Warning: in file [ROOT]/includes/captcha/captcha_factory.php on line 37: call_user_func() expects parameter 1 to be a valid callback, class 'phpbb_captcha_sortables' not found
Fatal error: Call to a member function get_demo_template() on a non-object in C:\htdocs\test\includes\acp\acp_captcha.php on line 124
The only way to fix this by the user is to manually edit the config table and change the captcha_plugin value to phpbb_captcha_gd (or other available plugin).