Unfrequent but unexpected popup "file changed" when saving a paired notebook with Jupyter-Collaboration enabled

[mwouts]

Sometimes (not frequently) I get this message when I save my notebook. This is unexpected as I know that I don't have modified the notebook outside of Jupyter. This did not happen before I installed jupyter-collaboration.

File Changed
"notebook.ipynb" has changed on disk since the last time it was opened or saved. Do you want to overwrite the file on disk with the version open here, or load the version on disk (revert)?