Drupal makes use of periodic cron operations to perform some house cleaning tasks to keep your Drupal installation healthy. One of the things it likes to check is for stale managed files. Managed files are ones that are uploaded to Drupal, where Drupal keeps a record of those files in the database, for example so that they can be referenced from nodes or users.
Some managed files are temporary, and so to optimise resource usage Drupal will remove those files from the database and from the filesystem once their expiry time has passed.
While trying to delete a temporary file something went wrong. The most likely cause is that you have a file permissions problem that is preventing your webserver from deleting this file, and possibly others from your filesystem. It is also possible that somehow an invalid URI for the file has been stored in the database, but this is less likely and might only be seen if you have some contrib or custom module that is badly behaved.
Try checking file permissions on sites/XXX/files an ensure that the webserver has appropriate access to files.