VMG8924-B10D, scp/sftp failure: corrupted IPv4 header if any ethernet device plugged

Here is a problem I described in Ubuntu forum ( https://ubuntuforums.org/showthread.php?t=2489877 ), and also reported by another OVH ISP customer (https://community.ovh.com/t/transfert-ssh-reseau-local-zyxel-vmg8924-b10d/38845/4):

When I try to transfer non-tiny files between two machines on my LAN, using scp or sftp, using IPv4, then if there is any machine plugged on an ethernet port of the router, then the transfer hangs at the very beginning. It works if the two machines are on wi-fi and if there is no ethernet machine plugged (involved or not in the transfer); I checked several times as it seems really weird. It also works if I use IPv6. I tried with multiple RJ45 cables and on all the Ethernet ports.

I looked with Wireshark and I could see that the client machine receives corrupted IP headers (and also packets which starts having a target IP `1.0.0.2` in the middle of the TCP communication). I can see that the other machine try to do some retries on the packets (without success).

I also used my phone as a local router instead of the Zyxel, and I got no problem.

I have a work-around (forcing IPv6 for ssh/scp/sftp between my local machines), but that's not satisfying: there could be other problems at any moment. I don't know when this problem started (I did not transfer large files from quite some time).

Firmware version: V5.13(ABGQ.6)C0