This can be caused if the user trying to log into SchoolPress has a missing or bad User ID. To test this, do the following while in the SchoolPress dashboard:
 
  1. Navigate to My Sites > Network Admin > User  User-added image
  2. Search for the user
  3. Hover your cursor over the user and click Edit
  4. At the very bottom of the screen the User ID can be found for the Podium User Info (this can only be changed by a SchoolPress Manager)
  5. Click Update User