IOTA rolled their own crypto. As they were warned by many in and out of the crypto (in both senses of the word) communities at the time, it was susceptible to attack. MIT Media Lab wrote a responsible disclosure report on a vulnerability. IOTA kept pushing back the publication date, based on a mixture of amateur cryptography arguments and nitpicking over disclosure issues. MIT Media Lab finally published, well after the originally agreed deadline, after realizing IOTA was not serious about improving the code further and was using the time to attempt to improve their appearance in the disclosure report's narrative. IOTA got upset, as this publication exposed them with their pants down. They've now threatened the involved academics with legal action. (There is, of course, no actionable basis for such.)

It is interesting if you find cryptocurrencies interesting, as it shows how "well" run many are - and IOTA is by no means an outlier here. The foundations are simply not lousy with cryptography, programming and economics experts who are solely interested in best practices.

Basically they are claiming one of the core primitives to make IOTA work as a secure cryptocurrency is fundamentally flawed. Haven't read enough but if this is what it sounds like, it means IOTA needs a major rework.

I mean what did you expect from a weird shitcoin that uses trinary arithmetic (really?) for no good reason.

edit: letter #11 says "shit's fucked, yo".

Because IOTA is rekt, it has been for some time, and this is just what you should expect from people who try to roll their own cryptography, then get defensive when a notable real cryptographer points out flaws.