Error thrown when there are cycles in system groups in 6.4 and 6.5 versions. Issue can also be avoided by not referencing the same group in update in group attributes.
Unity 6000.2 has reached its end of life and will not receive further updates. If you need additional fixes, please upgrade to a newer Unity version.
How to reproduce:
1. Open the attached “IN-122926” project
2. Close the project
3. Delete the Library folder of the project
4. Open the project
5. Observe the editor
Expected result: Project opens successfully
Actual result: Editor hangs while opening the project