PixieN wasn't that the document produced by JVL or something similar? 
Anyway, I don't think it's "proof that JC doesn't have an antisemitic bone in his body" unless by "proof" you mean "propaganda" 
EntropyRising I am afraid you are wrong about what IHRA. It merely differentiates between legitimate and illegitimate criticism of Israel, rather than stifling all criticism.
What it says is essentially that demonisation, delegitimisation and double-standards are antisemitism, but not criticism of the nature that you would apply to any other nation state. It's really not controversial - which is why, for example, the police and government use it for their definition of antisemitism.
I don't think JC is an antisemite himself, as I think he is one of a very, very few anti-Zionists that aren't (most are, because when you drill into it, it's actually only the self-determination of Jews they oppose, not borders and nations everywhere, to over simplify enormously) but, no doubt because of his Internationalist stance, he hasn't come down on the enormous amount of endemic, institutional antisemitism that pervades many layers of the Labour Party. This is a serious problem.
If the Chief Rabbi has issues - I believe him. In the same way, if Harun Khan said he had a problem with Tory Islamophobia or whatever, I would believe him too (though I am aware that many British Muslims feel that the MCB only represents a certain tradition of Islam, rather than all Muslims, no matter the intention when it was created