Error when opening file

I’ve been getting the following error message when opening a Scrivener file on my Mac OX El Captain. Other files open and convert fine to the new version of Scrivener. There is plenty of hard drive space. Any suggestions? Thanks

Error: The update has been cancelled because there was an error when trying to create a backup of the project. Please check there is sufficient space on your hard drive.

The message is a reaction to a general error where some condition made it impossible to write the backup. This could be permissions (very liable to break when updating an OS X directly over itself) or perhaps a sandboxing issue, if you are using the MAS version. There was a slight problem with it on release day, make sure you are updated to 2.7.1 if applicable.