I think it's unarguable his failure to challenge the A/S BS - more than that his legitimisation of it and willingness to expel people from the party on the say-so of zionist fanatics - has set back the Palestinian cause. There were welcome pledges in the manifesto to recognise the state of Palestine, stop arms sales to Israel and 'an end to the blockade, occupation and settlements', albeit couched in both-sides-bad language (ctrl-f for palestine here: https://labour.org.uk/manifesto-2019/a-new-internationalism/ ) but I don't know how much that differs from previous manifestos, especially in the promotion of the 2 state solution which has been dead for decades... In any case the resulting hostile atmosphere towards pro-palestine campaigners, the turning of analysis of zionism into a thought crime and the ongoing witch hunt against dissidents has massively emboldened the Israel lobby and made it much harder for people to campaign for genuine justice for the Palestinians.
There's no way Corbyn could have failed to notice that Israel supporters cry antisemitism at every criticism of Israeli atrocities. It would have been easy to draft a rebuttal and explanation of this devious tactic for him to state in his calm, reasonable way in the predictable instance of the accusation coming up, but he never did, and in fact drummed people out of the party for making those exact points in his defense. 'If you don't run they can't chase you' as I think Galloway said - face it down once or twice and it's over, but he was too, what... cowardly, stupid, masochistic, persuadable? to do so.