The Indian defence forum narrative is that on board electronics got damaged. There is no damage to Nuclear Reactor or the compartment ( which Indian navy has got no access to).
Word is Indian Navy messed up this boat big time. The explosion that caused the return of the ship before the lease was up apparently seriously damaged both the inner and outer hull. Indian navy patched up the outer hull enough that it could sail back to Russia on the surface and hand the boat back and pretend nothing happened. But now Russia is demanding $1.5 billion USD in damage.
I'd like to know what exactly is the issue and how the damage happened. India's handling of nuclear subs recently isn't a good one.