Hi,
some of my students are having trouble when logging into appinventor. They are using their educational google accounts and all they get is some kind of non valid URL. The problems is somehow spreading and don't know what's related to.
try trouble shooting and check internet connection and check if school administer blocked the site. if you still had problem take a screen shot and mention it
This URL allows you to open AI2 to the Project List, without opening any project. http://ai2.appinventor.mit.edu/?autoload=false
From that list, you can request Exports of Projects without trying to open them.