This error message occurs when attempting to connect to the student machines.
"Can't get your active classes.
Something could be wrong with your user account or with app permissions.
Click this message to try again. (900)"
This error message occurs when the website Youtube is blacklisted. Vision for Chromebooks needs access to http://accounts.youtube.com which is part of Google Accounts.
The websites that need to be white listed for Vision for Chromebooks can be found here.
To add an exception for a blacklisted site through Google Apps for Education (GAFE) or Google Admin, go here.
* Look under Content and then "URL blacklist exception"