SAP PI SFTP adapter Exception received 3 , Getting failed

HI Experts,

I am working in a file to idoc scenario and while connecting to the source system third party, the sender SFTP channel fails and gives below error:

Error:Exception received 3 , Getting <filename> failed.

The firewall between servers are ok and we have similar outbound scenario where PI is able to drop files to this third party.

Any idea what could be the issue here?

Thanks