Hello,
Self is VERY restrictive. It says that the only trustworthy source is the domain itself. If the images are in a subdomain, they will not be loaded (don't know if it restricts paths, also)
The * says "load from everywhere"
So, my FIRST guess would be that the images and so on, that he is trying to load, are not exactly in the "same" place as the application. Also, if the site is http and he is trying to access is using https, it would block the images if using Self.
And, finally, what I was looking for.
Here: https://stackoverflow.com/questions/16627310/wkhtmltopdf-not-loading-local-css-and-images#16650685
It seems that if you add a "base" tag to the header, could solve the problem (it seems that is a problem of the executable not sending the information correctly to the server)
Seems really to be a problem of communication between the wkhtmltopdf executable and the server...
Cheers,
Eduardo Jauch