AppsLocalLogin.jsp gives blank page in R12

Hi Everyone,
Recently I faced one problem in R12 instance. I was not able to see the login screen after application URL. I got blank page on AppsLocalLogin.jsp or sometimes I got a message unable to autheticate the session instead of giving me username and password screen. Backend side everything is working fine. I did lot of head strikes and searched lot of docs but did not found any. I also compiled all the JSPs,cleared persistence logs etc. but problem did not get resolved. Finally after 5-6 hours hardwork I got the soultion , one of my collegue has written the update trigger on apps.fnd_user table as below

CREATE TRIGGER xyz_trigger
BEFORE INSERT ON UPDATE
ON apps.fnd_user
FOR EACH ROW
BEGIN xx_demo.launch_work_process (:NEW.user_id, :NEW.user_name);
END;

Simply, I disabled the update trigger and it works for me. AppsLocalLogin.jsp Page is opening fine.

But I could not understand how it happens. If anyone of you know the reason then please update me .
Thanks
--Guru