I'll answer the questions I can, but I think it was something faulty in Windows. I came back to work Monday morning and I had an error message that said The number of connections to this computer is limited and all connections are in use right now. Try connecting later or contact your system administrator.. Long story short, I called Dell Tech Support and the tech I spoke with had never seen the message neither had his engineer seen it. After two hours on the phone, we had resorted to re-installing Windows. My searches now work fine on the SME server from Windows.
Just for the record:
searching for what and how? SMB or PuTTY? SMB
SMB v1 set on W10? I don't recall what it was set at.
NIC negotiated speed downrated? Not to my knowledge
faulty NIC? I wasn't having any other network problems so I doubt that was it.
bad cable? Same answer as above
intranet QOS kicking in? I don't know, but doubt it.
managed switch downspeeding? I don't know, but doubt it.
different subnet? No, all on the same subnet.
network traffic congestion I didn't monitor that, but I doubt it, we have 15-20 computers in the building and our router is a SonicWall TZ 500, well capable to handle the traffic.
network block size? I don't have the answer for that one.
SquidGuard (IP not whitelisted)? I know nothing about SquidGuard.
One thing that I am shying away from for now, is signing in to my computer with a Microsoft account. I'm using only a local administrator account (after the re-installation). I'm suspicious (can't really prove anything) that signing in with a Microsoft account put some restrictions on my machine that I wasn't able to control.