Couple of suggestions:
1. Boot from your bootable backup disk and see if the connection problem is still present. If the problem is not present, it means something most likely changed in the main macOS disk. But if the problem occurs with the backup disk as well, then perhaps something has changed on the NAS or the Ethernet switch. Are you using a managed or unmanaged switch/hub?
2. Check if anything has changed on the NAS systems. New firmware might have been auto-installed. Or SMB password or allowed-access-lists might have changed.
3. Check hub/switch configuration if it’s a managed switch.
I finally got a chance to get in a full troubleshoot.
1. Problem wasn't isolated, same issue occurred on backup disk.
2. Verified no recent changes on servers.
3. It is a managed switch, brocade ICX6610. Settings are as they should be.
That being said, I did discover/uncover the underlying issue. There seems to be some form of incompatibility with selecting jumbo frames is system preferences/network. This has never posed an issue prior to now. I was always able to select MTU 9000, and operate accordingly. With 9000 MTU, various LAN addresses are inaccessible via all browsers, connection issues over SMB and AFP (finder will free while connecting, freeze while accessing/transferring, generally slow SMB and AFP transfers, mounted ISCSI volumes dropping, mounted SMB shares disconnecting seemingly at random.
As seen in the pictures, 9000 MTU will not autoselect, so I have to select the pictured settings to see Jumbo frames as an option. With the pictured settings, 1500 MTU selected, expected operation. Jumbo frames selected, the issues occur. When I select Custom, enter 9000 MTU changes will not save. When I select Custom, enter (the max) 10222 MTU changes will not save. (note on Custom: I am able to hit save, then apply in the network settings, however the changes applied do not stick. When I return to configuration, the MTU is reset to 1500.
With MTU set to 1500, it at least appears to be workable. I just discovered this issue, so have not been able to fully test. But I can access all LAN locations, SMB shares seem to be stable. SMB shares are underperforming, in limited trial, but have historically with 1500 MTU selected. So, I'd like to figure a way past this issue, and get back to optimal network settings, as some of my use-case is high-performance-dependent.
I will also note when the NIC's are plugged in, with no changes, literally unplug from this workstation, pluginto macbook pro, plugged into rackmount mac pro, no issues whatsoever. So, seems contained to customac only. I've tried two different Chelsio T4 NIC's, Two different (same model) T5 NIC's, and one T6 NIC, with the same results. NIC firmware is the most recent, Catalina approved. Most recent catalina kext is installed.
I'm seeing if it may be a chelsio issue, i've reached out to their support. But, curious if any further insight can be given. I wish i had access to another mac compatible 10GbE+ NIC, but can't get my hands on any other brand, due to current availability.