The issue happens when using an index shared across multiple servers:

Server1: Tries to initialize directory provider
Server2: Tries to initialize directory provider
Server1: Acquires lock
Server2: Fails to acquire lock, start waiting 2 seconds (hardcoded on DirectoryHelper)
Server1: Adds some documents, does a commit and release the lock
Server2: Manages to acquires the lock
Server2: Initializes the index in mode CREATE
Server2: close the index writer, all documents added by server1 are lost