It might actually be a problem for shared service that modifies a page outside your home, but a personal ad-blocker is equivalent to the scissors you used to clip the ads out of a magazine.
Which is a great analogy that shows how difficult the argument is to decide.
If you take a scissor to a reproduction of a piece of art you have not created yourself (and for which the protection timeframe has not ended yet), create snippets, and rearrange them, this can be considered an Urheberrecht violation[1]. No one would consider doing the same to a newspaper the same.
[1] The distinction here is relevant: German copyright is not like US copyright - it can't be sold away, you can only license out individual derived rights, and it also allows for protection against "abuse of a work", e.g. doing something with it the original creator disapproves of[2]. If that always applies to the removal of ads has not been decided in a final decision.
[2] "The author has the right to prohibit any distortion or other interference with his work that is likely to jeopardize his legitimate intellectual or personal interests in the work." Art 14 UrhG
