Scattered thoughts:
From Barty's original post, (1) and (2) are true - clearly there is an issue on the left around anti-Semitism, for the reasons Padraig outlines. And it's not new. From the Jewish people I know, anti-Semitism has increased recently, and how could one not hold Corbyn partly responsible for that?
I think the thread contains a false premise though, (3) - "our outrage would be far stronger had the issue centred around gay people, muslims, black people, etc". I don't see much outrage over discrimination in general - most of it goes unnoticed because it is so much a part of societal fabric. Racism. homophobia etc are endemic in this society, and only the absolute most egregious stuff is even commented upon*. And racism and discrimination function in different ways against different groups - there is not straight equivalence. This seems obvious.
Also it seems obvious that these issues cannot be separated and pitted against each other, as though they were not interconnected and many people do not receive discrimination on multiple levels - for a start, many Jews are not white, and so experience multiple levels of racism.
The 'I'm going to vote for the Lib Dems rather than Labour because there is racism within Labour' is a headscratcher too. Voting for laws that oppress poor people (who disproportionately fall into discriminated-against groups, of course) through sadistic austerity doesn't count? It sure as hell matters to poor Jewish people, for example...
*e.g. to turn your point on its head, the stuff I heard said openly at my work yesterday about gay people (not even to mention the incessant coded racist comments vs Meghan Markle...) - had 'similar' things (and of course the point is that there IS no direct mapping) been said against Jewish people, might have resulted in a call to the police, and certainly a sacking. Discriminations simply do not work in the same way, and so blunt comparison leads nowhere.