Ash: /usr/libexec/sftp-server: not found when using scp

This is a consequence of your client machine using a very recent OpenSSH release (9.0 - check for more info), which changes the scp program to use the SFTP protocol under the hood, which vanilla OpenWrt/dropbear installations do not support. To work around the problem on the client side, use the new -O (that is an uppercase letter "o") switch when invoking scp, which will cause it to fall back to the legacy behavior.