Thank you everyone for the help!
Yes, I have entware installed, so that "caused" my problem.
However, for my simple task the "dumber" find command is more than enough. I finally used the -name function:
#!/bin/sh
DIR="/mnt/NAS/IPC_FTP/"$(date -d @$(( $(date +"%s") - 86400)) +"%Y-%m-%d")"/"...