He basically accused many players of cheating without any real evidence. His proofs are notoriously bad. The most notable examples are Daniel Naroditsky who was under immense psychological stress due to the allegations and died due to an accidental kratom overdose (kratom is used to self-medicate anxiety). Another prominent example are his allegations against Jose Martinez (aka Jospem) which resulted in them playing a grudge match which Jospem won. Kramnik was struggling with the tech in the online portion of the game. Basically he cannot accept that younger players beat him in quick formats and flings around ridiculous stuff. He's also feuding with Hikaru who is obviously really good at blitz.

I’m not a follower either, but inference from the article tells me: Krammik has come up with some sort of cheat detection method, has loudly accused some others of cheating, and FIDE are both unconvinced of the truth of the allegations and very unhappy he didn’t do this through proper channels. He also doesn’t appear to have co-operated with the investigation.

He posted mathematically unsound methods that accused online players of cheating. Including Naroditsky, who is no longer with us (draw your own conclusions).

This is a case of a Kramnik being a crank whom most people did not take seriously, but he still had an impact on the victims.

Fro Kramnik this is a mild penalty.