I see this exact same error message. The path for invoices seems to miss the correct file permissions Safe Path. AND it does go away with using the posted gavick method into the /log/ folder.
However, isn't the idea for the Safe Path to create a folder entirely outside of the public_html or root area of the site. Thereby, completely eliminating any one from having any possibility of browsing to that location?
Is there maybe a folder or file permission setting which needs to be set for the folder created outside of the public_html root?
Thanks.