The claim that anti-semitism is hidden under anti-zionism

both is true: Anti-semitism under the pretext of anti-zionism and legitimate critizism labeled as anti-semitism. Jewish people are instrumentalized by both supporters of the Israeli governent and anti-zionists.

but it is impossible to have a proper debate about it and differentiate these two in Germany

not impossible, but people don't pay attention to those who have nuance opinions. If you speak German, a good example would be this podcast by Jung und Naiv.

By blaming all anti-zionist positions to be anti-semitic, it also creates a reaction of defiance

I agree with that. In the same way, denying that anti-zionism and anti-semitism interconnected is equally unhelpful