When adding a new relation in INSERT mode, all existing relations are prepared to be edited (lock + check available action) whereas there is no reason to edit the existing relations.
Issue in _prepareSynchronizeContentReferenceMetadata method.

Use case :

• I have a content C which contains 2 children C1 and C2
• C2 is locked by another user
• I add a new child C3 (INSERT mode) to C

The relation failed because C2 is locked. The new relation does not affect other children so there is no reason to fail because C2 is locked.