Open Transaction that won't go away SPID Ns
I guess this is a known issue and Microsoft has released information about it in below links

Cause:

This issue occurs because the "tran_sp_MScreate_peer_tables" transaction was left open by a replication upgrade script during the upgrade. This open transaction prevents usual log truncation. 

Transaction log for database is growing with system SPID as open transaction

There is also a documented support article which will help you in understanding the issue

Database transaction log continues to grow after upgrade