Cannot add participants to my course

Summary

After going to the menu 'Participants', button 'Add participant', when whatever I fill in for the username, I always get 'Oops, ...'. I tried it with two different NetIDs.

Steps to reproduce

1. Go to Participants
2. Click 'Add participant'
3. Fill in a NetID (for instance 'rghorbani', or 'anujsingh'), set ID to 'TA', and 'Add Participant to course'
4. Results in 'Oops. Something went wrong on our end...'

Possible Fix

Logs