line 145 is trying to delete the temporary file that was created in the temp directory. Try setting the permissions on the temp directory to 0766.
You can also try changing line 145 of the WA_DigitalFilePro/HelperPHP.php file from:
unlink($attemptedServerFilePath);
to:
chmod($attemptedServerFilePath, 0766);
unlink($attemptedServerFilePath);
It is possible that the server is configured in a way that would not allow you to modify permissions on the temp folder, if you are still getting the same error with the correct permissions, you may need to work with the host to resolve the problem.