The SELECT would examine more than MAX_JOIN_SIZE rows; check your WHERE and use SET SQL_BIG_SELECTS=1 or SET MAX_JOIN_SIZE=# if the SELECT is okay
SELECT c.id, c.title, c.image, c.image_big, c.content, (SELECT COUNT(com1.id) FROM aqua_competition_comments com1 WHERE com1.work_id = c.id ) as com_col, c.author, c.`rating`, -- cv.ball, (SELECT COUNT(id) FROM aqua_competition_voting cv WHERE cv.user_code = '1556289351.1454426988' and work_id = c.id) as ball, (SELECT COUNT(id) FROM aqua_competition_voting cv2 WHERE cv2.user_code = '1556289351.1454426988') as ball_count, (SELECT SUM(j2.ball) FROM aqua_judge_competition j2 WHERE j2.work_id = c.id )as jball FROM aqua_competition c -- LEFT JOIN aqua_competition_voting cv ON cv.user_code = '1556289351.1454426988' and work_id = c.id LEFT JOIN aqua_competition_comments com ON com.work_id = c.id LEFT JOIN aqua_judge_competition j ON j.work_id = c.id GROUP BY c.id ORDER BY c.prioritet DESC LIMIT 0, 16