Update: although this hasn’t been fixed yet, we know have a temporary solution for this.
The system throws an error when the course creator is in the list of contributors. To fix this, simply remove the course creator from the list of contributors, save and then try again.
Please rest assured that doing so will not remove any permission the course creator has.
Yes, some time ago, someone on the team kindly found this issue and actioned the solution for a course I wanted to be a Contributor on and the Creator could not add me.