Logging in with Facebook OAuth results in the following fatal error:

E_ERROR: Uncaught exception 'OAuth\Common\Http\Exception\TokenResponseException' with message 'Failed to request resource. HTTP Code: HTTP/1.1 400 Bad Request' in /forums/vendor/lusitanian/oauth/src/OAuth/Common/Http/Client/StreamClient.php:68
Stack trace:
#0 /forums/vendor/lusitanian/oauth/src/OAuth/OAuth2/Service/AbstractService.php(183): OAuth\Common\Http\Client\StreamClient->retrieveResponse(Object(OAuth\Common\Http\Uri\Uri), NULL, Array, 'GET')
#1 /forums/phpbb/auth/provider/oauth/service/facebook.php(72): OAuth\OAuth2\Service\AbstractService->request('/me')
#2 /forums/phpbb/auth/provider/oauth/oauth.php(557): phpbb\auth\provider\oauth\service\facebook->perform_auth_login()

After some debugging, I'm able to locate the cause of the problem. (see /forums/vendor/lusitanian/oauth/src/OAuth/OAuth2/Service/Facebook.php line 161)

The new Facebook response is in JSON format, while the pre-2.0 API returns jquery string.

This bug can be fixed when you run composer update. (The lusitanian/oauth package has fixed it.)
However, the fix is not included in the latest 3.2 download.