cannot manage plugins

recently in my website http://asian-aroma.com/ i cannot install or uninstall any plug-in. i have some deactivated plugins but if i try to delete from the ftl, then the plugin management screen dissapears , i see white screen. if i copy egain the directory of the plugin works again. the strange is that the site works even i cannot see the plugin managment screen. i guess there is some problem with the directory /plugins on the server or probable some other problem . i changed themes also but the problem remains , independently of any theme i use. any help please ?