Try disabling windows defender, really. Especially "controlled folder access"
So Windows Defender (Antivirus) is already disabled, as I'm using Avast... Just for fun I tried disabling Avast... no change. Thought you might have meant Windows Defender Firewall, so I tried disabling that as well... no change.
What's interesting is, when I restart after making a change to settings and get the "path cannot be found" error I click cancel rather than diagnose... if I keep trying to browse the network shares right away, I keep getting the popup. But if I wait a minute or two before trying again, it works like it's supposed to. Could it be a network service set to "delayed" or "triggered" start causing the problem? My first thought was "DNS Client", but Windows won't let me stop/start/restart or change any of the settings on that one for whatever reason.
No, you said you had it connected to your home network which is a reasonable thing to do for testing a client's machine.
I suspect that most of us who contributed to this thread did so in a spirit of helping out a fellow technician who was in a spot of trouble with a client's computer - that's absolutely fine and we do it every day. But asking us (collectively - not me personally) for help with your own computer without making it very clear that it is your own computer is a lot more like asking us to give free assistance to an end user.
This probably comes across as a bit tetchy (and maybe it is), but you said yourself that you're only spending this much time and effort on it because it's your own machine and that you wouldn't do this for a client. Well, guess what? Neither would I. By now that machine would have been nuked and paved or on its way to landfill.
If you feel like I've mislead you, I apologize. But please don't think that I'm coming at this from an end-user standpoint, because that is not the case at all. If I just wanted the problem to go away I could have handled that on my own. But because this is something I've never come across before, and it's a problem that I may run into in the future with one of my business clients, I feel that it's important to at least
try to identify the problem and actually correct it rather than give up and nuke & pave. For most of my clients, that's a last resort that requires a full day of reinstalling LOB software and restoring critical data, all while impacting their production. So if I can work through the problem on my own, non-critical machine, and actually find the proper solution, it could save my clients a lot of lost time. Yes, a nuke & pave would be the quickest and easiest way to fix
this machine... but I'm not concerned with fixing this machine. I'm concerned with identifying the problem for future reference... for myself, and everyone else here.