I got this solved.
But here is the question and the solution.
General Error
SQL ERROR [ postgres ]
ERROR: null value in column "challenge_id" violates not-null constraint []
SQL
INSERT INTO phpbb_arcade_challenge (challenge_points, challenger_id, opponent_id, game_id, challenger_game_cost, opponent_game_cost, challenge_time) VALUES (0, 2, 55, 227, 0, 0, 1358351919)
BACKTRACE
FILE: (not given by php)
LINE: (not given by php)
CALL: arcade_msg_handler()
FILE: [ROOT]/includes/db/dbal.php
LINE: 757
CALL: trigger_error()
FILE: [ROOT]/includes/db/postgres.php
LINE: 211
CALL: dbal->sql_error()
FILE: [ROOT]/arcade/includes/class_challenge.php
LINE: 826
CALL: dbal_postgres->sql_query()
FILE: [ROOT]/arcade/includes/class_challenge.php
LINE: 1278
CALL: challenge->check()
FILE: [ROOT]/arcade.php
LINE: 134
CALL: challenge->random()
SOLUTION:I had to edit the challenge_id column edit defailt value (there was nothing in it). This is found in the Control Panel at Lunarpages PhpPgAdmin
nextval('phpbb_arcade_challenge_seq'::regclass)
Sorry for the edit.