This is a reference to Inter-Universal Teichmüller Theory. Its Wikipedia article gives a good overview (https://en.wikipedia.org/wiki/Inter-universal_Teichm%C3%BCll...). In maths lasting disagreements over a published "proof" are rare, but IUTT is an example of it. What the article misses: There is a more recent, ongoing effort to formalize the published proof in Lean under the name of "LANA" (e.g. see https://zen.ac.jp/news/zmcpostevent0717e and https://github.com/katobungen/LANA_report_202607/blob/pdf/LA... for a recent update). I guess most mathematicians agree that a successful compile of the proof in Lean would confirm its validity. My personal impression is that the process got stuck at the very point Peter Scholze and Jakob Stix pointed out 8 years ago. Officially LANA has still not reached a conclusion.