Cada tabla derivada debe tener su propio alias:error de combinación descendiente de MySQL

En este caso, la tabla derivada que requiere un alias es aquella de la que está SELECCIONANDO *.

La sangría ayuda a que eso quede más claro.

SELECT * FROM 
(
    (SELECT '1' AS `table`,  `vid_req_timestamp` AS `timestamp`, `title` FROM `movies` WHERE `vid_req` = '1')
    UNION
    (SELECT '2' AS `table`,  `ost_req_timestamp` AS `timestamp`, `title` FROM `movies` WHERE `ost_req` = '1')
) AS `some_table_name_lol_this_is_an_alias`
ORDER BY `timestamp` DESC