That is due to the changed default of the underlying transfer protocol in the newest scp versions to be actually SFTP. If you have a new scp version in your Linux, PC, Mac, whatever, you need to either
- use "scp -O" option to force the old "legacy" transfer method instead of SFTP
- install openssh-sftp-server to OpenWrt (to provide the SFTP server functionality in the router)
See info e.g.
- https://www.redhat.com/en/blog/openssh-scp-deprecation-rhel-9-what-you-need-know or
- https://www.cisco.com/c/en/us/support/docs/troubleshooting/220371-scp-from-clients-on-openssh9-0-to-ios-xe.html
and