Hello @CristinaKrysins
Thank you for contacting the Instructure Community. We're sorry to hear you've been experiencing issues with grade passback. That error message is a bit abnormal and it may be there are “headers” Invalid character in header content [“Authorization”] - essentially suggesting that they have misconstructed this authorization header in their request. Which also means- you should speak with Canvas Support directly. This is difficult to resolve without looking at the specifics of the course, assignment groups, specific SIS you use, etc.
Because of the complexity of the issue, and the concerns for privacy, it would be best if you contacted Canvas Support directly to pinpoint the source of the issue and provide a resolution:
How do I contact Canvas Support? - Instructure Community
