Edit 2020-08-26 : End of the story, the issue has been fixed in the Debian repos.
This is related to this forum post https://forum.virtualmin.com/t/impossible-to-connect-in-sftp-since-lates... but I allow myself to create an issue since it really looks like a bug.
ProFTPd has been updated in Debian 9 repos a couple of days ago. Since then, SFTP connections on port 2222 aren't possible anymore. I am unsure what is exactly on the hand since the logs don't give much clues.
My temporary workarround is to downgrade ProFTPd, doing so everything gets back on track.
This is easily reproducible :
1) Launching a fresh server with Debian 9, in my case a VPS at OVH: https://vps481644.ovh.net:10000 2) Install Virtualmin with the install script, evrything works like a charm 3) Create a top-level virtual server "vps481644.ovh.net" 4) Trying a SFTP connection on the 2222 port for the user "vps481644"
It fails with a vague error message and not much more in the logs.
5) Downgrade ProFTPd: "wget http://vps481644.ovh.net/proftpd-basic_1.3.5b-4+deb9u5_amd64.deb" (that's the previous version) + "dpkg -i proftpd-basic_1.3.5b-4+deb9u5_amd64.deb"
And SFTP is working again.
I run a lot of Debian 9 server and VPS ... a little help would be greatly appreciated :-)
Best ragards, Nico