This issue happens when a user is trying to modify a file that is locked by another user. This may happen when locking status has not been synced to the local client while the user modifies the file. Meaning while someone is clicking on file locking someone is editing it.
To see such syncing issues you can click on the status bar icon and then 'view syncing errors'.
If you have MAC it's on the upper left corner, 'File' and then 'view syncing errors'
See picture attached:
Then you can find which files have been locked. Then you need to unlock these files via a web interface.