MudhornDescriberCapabilities warning after submitting

Yesterday I submitted several ebooks to the Apple Book Store. They were accepted, yet at the end of each submit process I got also this warning.

Contacting Apple Support I got an answer as funny as the warning: “hello, (…) I see that your books were succesfuly accepeted”.

Googling for Mudhorn Describer Capabilities one gets this stackoverflow page showing that the reason of the warning has not been found yet.
Anybody knows anything about it? Because I have a feeling that a third party is interfering during our submissions to Apple. Yet Apple, as I said, does not give a clear answer.