When you log in the default action is to return you to the index page, it doesn’t sometimes choose the index, home page ,sometimes wp-admin.
It sounds as though you have something altering the default behavior a plugin? if so same actions apply disable all plugins until the issue resolves itself, now you know which plugin causes the problem – although in this instance it may well be apparent that there is a specific plugin in use but only you know that , third party plugins are very difficult to support and issues must be addresed to the plugin author.
