After updating my PR4100 to MyCloud firmware version 5.22.113, Resilio Sync no longer starts.
Upon looking at /shares/Volume_1/Nas_Prog/ResilioSync/start.sh we can see the following line:
USERNAME=$(grep '/home/admin' /etc/passwd | cut -d ':' -f1)
however the /etc/password no longer contains /home/admin
changing this line to the following solves the problem and the Resilio Sync background task starts OK
USERNAME=$(grep '/home/restsdk' /etc/passwd | cut -d ':' -f1)