See https://www.math.columbia.edu/~woit/wordpress/?p=10560, which links to a technical write-up by Scholze and Stix about what they think is the issue with Mochizuki's proof. Woit's blogpost also gives a bit more links, including a response by Mochizuki.

The paper which everyone linked to when the discussion was going on now 404s, so here is a new link: https://ncatlab.org/nlab/files/why_abc_is_still_a_conjecture...

The point in the proof where scholze and stix show it fails is “IUTT-3, Corollary 3.12”