Up until now, there was no option to disable these log entries, however, I have added an option in tonight's release of Omniscope 2.6 under "Settings > Advanced > Miscellaneous" called "Disable invalid URL loggings".
If ticked it will disable logging of such errors in the server log.
Please not that my solution above, may well be hiding an underlying problem. From what I can imagine, I believe the problem may have something to do with unable to find a particular image probably because of an invalid URL to an image?
The files are stored on a secure server so when they are opened online the user is already logged in and the files download. When running it through the schedule it is not logged in hence the error.
Thanks for the fix - I will "tick the box" when we go through our next upgrade cycle.