Thought I would share this in case anyone else has the same problem.
My old and trusty ZyXel NSA325v2 had been happily syncing for a few years after I upgraded from btsync 1.2.73 to btsync 2.2.6 in 2015, until, it seems about May 2018.
Because Resilio Sync "just works" i didn't noticed it had stopped until I tried to pull down a selective sync folder. So in September 2019 I upgraded to the latest version, with the help of ResilioSync support.
The latest version, 2.6.3, is NOT supported and will not work. so you need a slightly older version (2.5.12)
Download the ARM version https://download-cdn.getsync.com/2.5.12/linux-arm/resilio-sync_arm.tar.gz
Unpack the tarball `tar xvzf resilio-sync_arm.tar.gz`
Copy `rslsync` to your NAS drive (FTP, SSH?)
Telnet/SSH to your NAS, copy `rslsync` to the folder: `/usr/local/zy-pkgs/bin` but name it something like `rslsync_2.5.12`
Move the old binary and replace it with a symlink `mv btsync btsync_2.2.6` and then symlink the new one like `ln -s rslsync_2.5.12 btsync`
Reboot the NAS simply enter the command`reboot`
Hope this helps someone!