Peer-to-peer networking

Prior to my recent vacation, I was sharing data on other computers in my peer-to-peer network.  A principle application was to synchronize data on my MacBookPro - 2017 (MBP) machines to a MacPro - 2012 (MP) with 8TB of storage. I use one of my MBPs as my principle computer and sync to the MP shared directories nightly. The MP is mounted on the Primary MBP, or can be mounted by the sync program.

Problem

After returning from vacation, I am no longer able to connect from MBP 2017 with MP 2012. All efforts to connect fail.

I can still connect from MP 2012 with MBP 2017.

What is going on here?