Win10 can't connect to Win8.1 but Win8.1 can connect to Win10

(1) That is pretty nice to hear. I hope it will last or that, as you say, you'll just have to redo it after the Win 10 does a reboot. But what is the "file sync software" on the Win8.1? Are you sharing the files & folders through OneDrive in some way? It does seem to confirm it is something about 8.1 causing the problem, even if it is triggered by a reboot of Win 10. 

(2) I kept clicking at that link I posted earlier & got to this...

https://support.microsoft.com/en-us/windows/file-sharing-over-a-network-in-windows-10-b58704b2-f53a-4b82-7bc1-80f9994725bf

File sharing over a network in Windows 10 I thought the section "How do I troubleshoot sharing files & folders?" is interesting, especially the part "Make sharing services start automatically". It's reminiscent of that FDResPub thread. But I don't know whether it will make a difference or is even worth messing with at this point. I hope what you have done will last.

>> what is the "file sync software" <<

The software is Free File Sync and while it's design, functionality, options and capabilities are excellent, imho, the "Free" and the payment options are misleading, so, I am not recommending it.  It is better than any of the many other file sync programs that I have researched and/or tested.

That, however, is moot.  My observation is that any file transfer action such as manually transferring a file using file explorer from the Win8.1 computer would have probably triggered Win10 to allow connections in the other direction.  Again, I am now more certain than ever that this is a user rights glitch within Win10.

I'm guessing that there are not that many users out there that don't login with admin rights if they have that option.  Logging normally in normally with non admin rights does give one at least a tad more sleep at night ;-)

Beverly

It sounds like Free File Sync is a backup app akin to Windows's own File History, but it makes single copies instead of keeping a record of every change. You only get to restore the last version of each file which is less complicated. Seems to have good reviews, sorry to hear you must pay. I doubt it has a bearing on the issue at hand, unless it is set to sync Win 8.1 files to Win 10 files across the network. But that's what sharing across a network is supposed to do all on its own, I think. Or am I wrong? You know better, really. But, even if it requires syncing, Free File Sync likely isn't the cause of the problem. It is just the canary in the mine that dies first when the network is broke.

I only have an Admin account. I believe a Standard account is safer, but I don't want to be constantly bothered with UACs & lesser user abilities, So, I trust in Windows Security for protection. Also, I keep full system backups in case something does get in & cause real havoc. But neither do I have a Win 8.1 computer, nor even a network. So, I can't say whether the problem would happen to me. I've run this way since 2014 & never got a virus (but I never go anywhere suspect).

(a) Why don't you make your account(s) an Admin for a month -- just to test your theory?

(b) Why not file share through OneDrive one way or another instead of the network?

(c) Have you tried a P2P Network as Kate LI (MSFT) wrote in here...?...

https://social.technet.microsoft.com/Forums/windows/en-US/492f0a6a-13ce-47ea-9118-7922b0c3a50d/windows-10-peer-to-peer-network-setup

Windows 10 peer to peer network setup

Sounds easy enough. Well, get to the Workgroup name this way: "Right-click START, System, System Protection, Computer Name tab"...

I don't know how well it would go or whether it's worth trying seeing as you've got a work around to the problem.

>> I don't want to be constantly bothered with UACs & lesser user abilities <<

In almost all cases, when logged in as a non-admin user, when an action requiring admin access, windows states that, and you can then select the admin account and enter the admin password... I've gotten used to it.

Again, the strange thing is that this problem exists when I am logged in as a non admin user on the Win8.1 computer... and I have no problems accessing the shares on the Win10 computer FROM the Win8.1 computer.

>>

(a)

(b)

(c)

<<

I'm done... I seem to have a semi workable solution... you would think that someone from ms would at least consider addressing issues such as this one... perhaps it's time to start learning more about linux ;-(

Thanks for the response.

You are welcome. OK. I think you'll be fine with the workaround you discovered. Good going. Sorry I was no help toward an actual solution. But I think you are right, the onus is on Microsoft -- it's on their heads! Good luck, & keep us informed.

Just to summarize, the problem is that Win 10 will lose its connection to Win 8.1 (but not the other way around). That is, Win 10 no longer sees the Win 8.1 computer in its File Explorer, Network folder, (but Win 8.1 still sees Win 10).

Then...?...

(a) Sign in to the Admin account on the Win 8.1 (then back to the Standard account, if you wish).

(b) Maybe sync the files from the Win 8.1 computer to the Win 10 computer?

After that, Win 10 can see the Win 8.1 computer in the Network folder, & its shared files are accessible. Nothing needs to have been done on the Win 10 side. It's not that tough a workaround.

Actually, it almost sounds like it is a design feature to protect computers that are logged into a Standard (non-Admin) account. It may be meant to protect file access against intruder prying eyes.