error retrieving class

I have not been able to view my class roster or my class data for one of my classes for several weeks.  I am  not able to see progress in their grade level mission work or any other time they have logged in Khan Academy. I can only see assignments completed that I have assigned. 

I have tried different browsers and tried clearing my cache and cookies.  I receive a message stating "error retrieving class" when I click to view progress or roster for this class. My  other class appears to be working fine.