How to reproduce:
1. Open the attached “CrashRepro“ project
2. Open the “Assets/Scenes/SportCar_Demo.unity“ Scene
3. Select any GameObject in the Hierarchy
4. Change the “X“ position
5. Save the Scene (CTRL + S)

Reproducible with: 6000.0.58f1, 6000.2.5f1, 6000.3.0b2, 6000.4.0a1

Reproducible on: macOS 15.6.1 Sequoia (Intel), Windows 11 (by user)
Not reproducible on: No other environment tested

Note: Disabling the “Spline Computer“ Components in the “OutsideTrackColliderSpline“ and “InsideTrackColliderSpline“ GameObjects in the Hierarchy will stop the crash from reproducing

First few lines of the stack trace:
0x00007FFF1B01D487 (Unity) CollectUsedSceneObjects
0x00007FFF1B01B5B1 (Unity) CollectAllSceneManagerAndObjectIDs
0x00007FFF1B02E247 (Unity) SaveScene
0x00007FFF1B322256 (Unity) EditorSceneManager::SerializeSceneIntoFile
0x00007FFF1B320064 (Unity) EditorSceneManager::SaveSceneAsAnAssetAtPath
0x00007FFF1B321DEF (Unity) EditorSceneManager::SaveScenes