Great - that really helped!
Removal of the Content-Type-line (which I must have overlooked when trying to improve a fix) helped for the Sony, it worked like a charm.
Concerning the Kobo, I'm not sure what the problem is; I stumbled upon the Kobo displaying the 2.1.5 version number but the 2.1.4 version information. Your hint at 2.3.0 being the current firmware got me to upgrade the Kobo, and now it works without further changes.
So again - merci beaucoup :-)