What is the cause of cracking and corrosion in Trent 1000 engines?
The problem is sulfidation corrosion, according to this article:
https://airinsight.com/rolls-royce-trent-1000-corrosion-issues/

Turbine blades are made of metal, and they suffer temperatures and forces so high that they are usually 'grown' from a single metal christal. A single blade can cost as much as a luxury car. 

The problems also seem to be related. Corrosion leaves a little pit, which turns in to a crack due to the cyclic loads.