Some time ago, I bought a Redmi Note 4x global version also known as "mido". I decided for this one because it is currently one of the most popular phones for Lineage OS and has a perfect price/value ratio.
Since a few month, I wasn't able to install any updates for my Redmi Note 4x China phone with Lineage OS 15.1. It always failed with the message unable to mount storage, even when I entered my correct passphrase to unlock it in TWRP.
Today I searched for the solution and the easy fix was to update TWRP to the latest version 3.2.3 which fixes exactly this bug.