Change Subgroup Parent

Hi All, I am trying to change the parent of a subgroup but I am getting Error Message "The website encountered an unexpected error. Please try again later."

The process I am using to reproduce

1. edit the subgroup
2. in the groups field, remove the current parent (remove and confirm)
3. Add the new parent
4. hit save.
5. Error

Am I doing this right or is there another way to change a parent?

Thank you,
Danny