Error messages using FTP sync with NAS server

[LLeynen]

I am using FFS (donation edition) to synchronize a number of folders on my desktop PC and laptop (Windows 10) with my Asustor NAS, using FTP. I configured the sync to use parallel file operations.

All the time I am getting error messages, see example screenshot in attachment,
"Cannot read directory "ftp://..."
CURLE_WEIRD_SERVER_REPLY: Failed EPSV attempt, exiting
425 EPSV: Address already in use [curl_easy_perform]

When I click "Retry" the process continues, but the error keeps popping up all the time...
Any idea what I can do eventually change the configuration to resolve this?

[Zenju]

The real issue seems to be "425 EPSV: Address already in use". As a consequence the FTP client tries to switch to PASV, but this isn't feasible for your IPv6 connection, hence "Failed EPSV attempt, exiting".
Maybe the port range for data connections is too small and facilitates "425 EPSV"?

[LLeynen]

Not sure what I can do to solve this.
It is related to the number of parallel file operations; when I decrease that number, the problem becomes less frequent, only when I set it to 1 no problem anymore.

FFS SYnchronization Settings.jpg (77.84 KiB) Viewed 465 times

On the server side, settings are as follows, I assumed this is sufficient:

FFS SYnchronization Settings.jpg (77.84 KiB) Viewed 465 times

[Zenju]

This looks like a fairly small range. Try increasing it, chances are, this fixes the intermittent errors above:

port-range.PNG (19.15 KiB) Viewed 454 times

If this doesn't work, maybe the issue is that your server simply doesn't support EPSV. In this case you need to access it over IPv4 instead of IPv6, so that the PASV fallback can kick in.