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 SchoolPress:
  1. Navigate to My Sites > Network Admin > User
  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