Files showing as missing when present

[gtandzi]

I've been trying to chase down a fairly recent problem with a sync process I set up a long time ago, and which consistently worked for a long time. Or perhaps it's two problems?

I'm syncing files from an HDD on my computer to a Synology NAS over a mesh network.

I open FreeFileSync and run Compare. It shows some files not yet synced. So I run Synchronise on the same comparison. It starts syncing, at about 10Mb/s. Then over the next minute it gradually slows, then stops altogether, usually at around 50-60%. Another 30 seconds or so, and I get a message:
ERROR_UNEXP_NET_ERR: An unexpected network error occurred. [FileCopyEx] (or sometimes [DeleteFile]).

And this happens for every file. I've tried many many solutions as found on the internet, but no luck.

BUT, if I log into the NAS itself, those supposedly missing files are there, and as far as I can tell, are identical. Certainly, if I try a straight copy and paste, it says the files are identical. And I can play them successfully (they're .flac files).

Would love some suggestions. Thanks in advance.

[therube]

And I can play them successfully (they're .flac files).

Playing successfully does not mean the files are identical.
Check them with a comparison/hash program.

(I don't really know what a "mesh network" is?)

If you copy a set of files from Windows Explorer to the NAS, do you see the same type of behavior (fast initial speed followed by slow downs, then extended delays)?

[xCSxXenon]

Being a mesh network, I suppose this could be caused by transitioning between APs, but that seems unlikely. Windows is, now at least, pretty good and resuming after a dropped network connection. As therube said, maybe the files aren't identical, it is definitely worth calculating a SHA-256 hash on both sides to check.

[gtandzi]

A firmware update for my mesh (TP-Link Deco) today fixed the problem immediately. Fingers crossed it's a permanent fix. Thanks for your suggestions