When save product after modified attribute combination there is a warning: "You can't edit the product because has been changed. Please refresh page and try to change again". If continue to click save, then "Product attributes" and "Attribute combinations" will could not add new or delete.

1. Edit product, Click Tab "Inventory", Change "Manage inventory method" to "Track inventory by product attributes", Save it.
2. Add some "Product attributes" and checked "Allow to use in attribute combinations", add some "Attribute value".
3. Modify (Add / Delete / Generate / Clear) "Attribute combinations".
4. Click "Save" or "Save and Continue Edit".