Indexing never completes

Using Client 2.1.16-cbt.
On one of my windows Servers the Indexing just runs forever and never completes.
For example, this started on the 29/04/2017 and has still not completed:
2017/04/29 07:32 DEBUG poscontrol1: Connecting for filelist (async)…
The Server is running Windows 2012 R2 64-bit.
Any help will be appreciated.

I have done a upgrade to version 2.1.18-cbt with the same results.
Anyone have any idea?
It seems like it is only happening on some Windows 2012 R2 machines.

Got some more info:

2017/05/02 15:56
Starting scheduled incremental file backup…

2017/05/03 12:49
Constructing of filelist of “poscontrol1” failed: error - Async indexing process not found

2017/05/03 12:49
Backup had an early error. Deleting partial backup.