Here the ss of Browser Console.
How to reproduce the error message.
1- Open any project, here I opened C3 default Kiwi Project
2- Save it as Project Folder (C:\Kiwi)
3- Firstly I changed something minor then hit the save button, It saved successfully.
4- Duplicated a layout and hit the save button, here the error messages pop.
5- After pressing OK (from the error message) and re-hit the save button, now it saves successfully.
Having said that I'm on Chrome 85 with Native File System API enabled.