Project root in Dropbox directory.

Symptoms:

1. Project is located in a folder shared via DropBox.

2. Created files disappearing.

3. Corrupted files without names appears. 

4. In the Project Structure tool window, non-existent folders appear.

5. IDE shows warnings that the file was edited externally.

Cause:

DropBox constantly makes operations with the assigned directory and doesn't allow other applications to read the directory correctly.

Resolution:

To mitigate most problems, switch Dropbox Preferences | Sync to Available offline.