I notice the warning message in the server error log last month, but after changing the hosting service this week, the warning is now displayed in Web pages containing a featured image. May be a host-specific issue depending upon how PHP is configured. FWIW.
What’s troubling most is that the warning contains the full location (including the root folder path) to WP-Includes. The root path and FTP user name might be (or “is”) same in some hosts.