Suddenly lost LAN file share access to local server

So this is a weird one. My desktop PC (Windows 11 Pro 23H2) has been working fine since I built it a couple months ago. It's hardwired to my LAN, and connects to my unraid server, and of course to the internet as well.

Yesterday, I suddenly lost SMB/explorer access to my server shares, but I knew the server was still up, because the cameras were working (they're hosted on the server) and I could log into the server GUI via a browser. I just couldn't browse any of the file shares. I tried rebooting (always step 1), which immediately fixed the issue, so I chalked it up to a typical Windows glitch and went to bed.

This morning, I got a message that my overnight backup had failed, and I have no file access again. I started digging a little deeper and found the following:

1. The Windows networking 'troubleshooter' reports that "The remote device or resource won't accept the connection". The tooltip says further: "the device or resource is not set up to accept connections on port "The File and printer sharing (SMB)". This is completely bogus, since I can access those shares from the file manager app on my phone without issue, so I know the server is just fine.
2. Simply running the troubleshooter does not fix the issue.
3. Disabling and re-enabling the NIC also does not fix it. I have to actually reboot the entire system.
4. I'm noticing an indicator of background activity that I have never seen before. What I mean is the little flickering circle icon next to my mouse cursor. This is a new, very fast workstation. I've never seen that icon appear when I'm just tooling around on the desktop or browsing the web. I feel like there's some new background activity that may or may not be related to this network issue. I've checked Task Manager, but I don't see anything obviously new or suspicious. I haven't installed any new software recently, except Skype a couple days ago.

Any ideas? This is a real pain in the rear.