I speak German legalese (don't ask) so I went to the actual source and read up on the decision.

The way I read it, the higher court simply stated that the Appeals court didn't consider the impact of source code to byte code transformation in their ruling, meaning they had not provided references justifying the fact they had ignored the transformation. Their contention is that there might be protected software in the byte code, and if the ad blocker modified the byte code (either directly or by modifying the source), then that would constitute a modification of code and hence run afoul of copyright protections as derivative work.

Sounds more like, "Appeals court has to do their homework" than "ad blockers illegal."

The ruling is a little painful to read, because as usual the courts are not particularly good at technical issues or controversies, so don't quote me on the exact details. In particular, they use the word Vervielfältigung a lot, which means (mass) copy, which is definitely not happening here. The way it reads, Springer simply made the case that a particular section of the ruling didn't have any reasoning or citations attached and demanded them, which I guess is fair. More billable hours for the lawyers!

[Edit: added "The way I read it, coz I am not 100% sure, as explained later.]