Giles Gardam's [counterexample to Kaplansky's unit conjecture][1] was a big breakthrough in group rings.


  [1]: https://projecteuclid.org/journals/annals-of-mathematics/volume-194/issue-3/A-counterexample-to-the-unit-conjecture-for-group-rings/10.4007/annals.2021.194.3.9.full