Server 2012 R2-DFS Replication not working one direction-Insufficient Disk Space error but it’s not

I have seen several posts on this issue and possible solutions, so far nothing helped in my case.

We have two servers S1 (Primary) and S2 connected in LAN. We have users folder in Replication Group. Replication Group's bandwidth is full. From S1 to S2, its working fine and backlog is very low which is normal. But S2 to S1, its stuck at 7779, an hour ago it was 7780. I have checked DFSR event logs, DFSR diagnostic reports etc.

In event log and also in the report, for S2 server, there is an error:

DFS Replication unable to replicate files for replicated folder Users due to insufficent disk space.  

  Affected replicated folders: Users 

  Description: The DFS Replication service was unable to replicate one or more files because adequate free space to replicate the files was not available for staging folder E:\Users\DfsrPrivate\Staging. Event ID: 4502 
  Last occurred: Wednesday, January 30, 2019 at 3:34:46 PM (GMT10:00) 

  Suggested action: Determine whether the volume reported hosts the replicated folder, the staging folder or both as in default configuration. See additional information about disk space under the informational section in the table titled "Current used and free disk space on volumes where replicated folders are stored". Ensure that enough free space is available on the volume for replication to proceed or move the associated replicated folder or staging folder to a different volume that has more free space.

Now, our S2-E: drive is 42.1TB and 28.3TB is free, S1-E: drive also has similar space. Users is the root shared-folder that contact individual users folders. Usually users files are not that big.

Users folder Stating Folder size has neber been a problem as I allocated sufficient space (200GB) for both servers. When I checked staging folder current size, on S2 its 4.49GB only, on S1 its 146GB.

When I run " Dfsrdiag.exe ReplicationState" on S2, it gives me this:

dfsrdiag.exe ReplicationState /member:S2

  Total number of inbound updates scheduled: 88

Summary

  Active inbound connections: 1
  Updates received: 120

  Active outbound connections: 0
  Updates sent out: 0

Operation Succeeded

For S1,

dfsrdiag.exe ReplicationState /member:S1

  Total number of outbound updates being served: 15

Summary

  Active inbound connections: 0
  Updates received: 0

  Active outbound connections: 1
  Updates sent out: 15

Operation Succeeded

Just a week ago, S2's replication service got into issue and it had to rebuild its database, then did an initial replication that took around 2-3 days to complete. Since then replication service is running fine. The most recent event log that catches my eye after S2 was last rebooted is drive E: free-space issue (Event ID 4502). Right before that there is another entry 5014
Text

The DFS Replication service is stopping communication with partner S1 for replication group RG01 due to an error. The service will retry the connection periodically. 

Additional Information: 
Error: 1818 (The remote procedure call was cancelled.) 
Connection ID: 257B85DC-8C09-42EF-9727-4176A2F88527 
Replication Group ID: 158FE127-1927-463F-88CC-70E6B0014656

This is I have, and I am in a loop in finding our out what is responsible for S2 not replicating or very very slow replicating to S1. Any advice/help will be appreciated. Thank you.