Hi again..
This time I will not complain.
Because the problem doesn't exists anymore!
While triying to identfy the problem I decided to disable all the plugins step by step whether to see if something will change or not. When I disabled "plg_system_expose" named plugin (which I later discovered is not compatible with Joomla 3.x) the Menu items division of Menu Manager worked again properly. Than I installed and enabled the plg_extral_params_for_menu_j25-j30_v30 and SJ-Mega_Menu again.. They also worked and now there is no problem..
But I was totally forgotten "plg_system_expose" plugin because it was installed on 22 June 2013 and arouse no problem till yesterday when I activated "plg_extral_params_for_menu_j25-j30_v30".
So, thanks anyway...