I didn't believe it either until I looked into the examples. It's extremely sad but the evidence is pretty damning.
http://jewishnews.timesofisrael.com/labour-alan-bull-council-holocaust-hoax/ (Labour were aware of this holocaust denier for a long time before they were eventually pressured into expelling him)
https://www.theguardian.com/politics/2018/mar/08/labour-suspends-party-members-in-antisemitic-facebook-group - a group dedicated to anti-Jewish conspiracy theories (including holocaust denial) involved plenty of Labour members, including Corbyn himself.
https://www.telegraph.co.uk/news/2017/05/20/jeremy-corbyns-10-year-association-group-denies-holocaust/ - more holocaust deniers with whom Corbyn was quite happily associating.
https://www.birminghammail.co.uk/news/midlands-news/labour-take-no-action-against-11923281 - another Labour councillor promoting anti-Jewish conspiracy theories.
https://www.thejc.com/news/uk-news/haringey-councillors-say-it-is-now-impossible-to-be-a-jewish-labour-councillor-1.460947 - two Labour councillors abused by the party's own members for being Jewish (incl. explicit racial comments) until they left office.
Gerry Downing was expelled from Labour for using frankly Hitlerite language about 'the Jewish question' and supporting 'Jews control the world' conspiracy theories (plenty more of them which I haven't posted).
https://www.thejc.com/news/uk-news/labour-suspend-activist-over-jewish-genocide-of-the-british-people-tweet-1.56965 - Labour party member suspended for claiming that Jews are orchestrating genocide in Britain, a bizarre variation on the usual 'white genocide' racist conspiracy theory but this time applied to Jews.
https://www.haaretz.com/jewish/u-k-labour-party-member-under-ifire-over-anti-semitic-posts-1.5425962 - Labour councillor and former mayor endorses the idea that 'Hitler killed six million Zionists' - which has plenty of possible interpretations, and all of them are pretty sick.
Josh Simons, a former policy adviser, has alleged that language of a 'Jewish conspiracy' was commonplace in Corbyn's office and in particular described hostility towards Jews from senior advisors like Seamus Milne.
Then there's the constant racist abuse of Jewish councillors and members like Adam Langelben, Luciana Berger, etc.
This is clearly a serious problem in Labour and did not used to be. I don't think there's any obvious evidence that Jeremy Corbyn himself actually hates Jews or holds anti-semitic viewpoints - but he is clearly not very fussed about letting other people who do run rampant in the party, and this *completely* undermines all the anti-racist protesting and activism he did as a young man. If you're not going to take action against racists when it matters, i.e. when you are actually in a position of power to do so, then it really does count for nothing how many protests you've been on or how many articles you've written or how many times you've been arrested. He's allowed this to go on and on and on in Labour and the actual anti-racists in the party have had enough.