But is 'rubbing tree' really a very recognizable collocation? Just grammatically, it could sound like a tree that rubs (against something else) or that is used for rubbing (against something else).
My impression is that many people are aware that animals rub up against trees or posts, or use them to scratch, but that we don't have a special word for those even in the literal sense, much less in the figurative sense. Maybe partly because there's nothing unique about a particular tree or post to cause an animal to choose it, apart from the fact that maybe some other animal has already rubbed against it or scratched it and left scent there.
I would also be cautious about expecting English speakers to perceive any implied connection between rubbing, which is usually used in the literal sense, and friction, which is the word in English that more typically can carry the figurative sense of conflict. In Germany both have the same etymological root, but in English there's simply no connection, they're different words.
So I would actually be inclined not to even attempt to translate too literally, but to substitute some other phrase, like, indeed, 'source of friction,' 'bone of contention,' etc. -- or indeed, the perfectly good other ideas in the original post.
In fact, I don't quite see why the German needs two words in the first place, when it already has 'Grundlage für hitzige Diskussionen,' which already covers the point and which could simply be translated. The idea of an author or an author's works as a kind of scratching post just doesn't work that well for me in the first place, so I wouldn't feel that bad about just quietly losing it.