Ubuntu – ssh connection refused Ubuntu 18.04 – Proxmox and OVH –

The first thing to do in this situation is to use the -v option to ssh, so you can see what types of authentication is tried and what the result is. Does that help enlighten the situation?

In your update to your question, you mention "on another local Ubuntu". Have you copied over the ssh private key to the other machine?

Which ISP are you using? That info might be useful as other people on the same ISP may be able to confirm the behaviour (as they have the same problem) or report all is well for them (indicating that the ISP doesn't have a general problem with SSH traffic).

One possibility if it is the ISP, is that they've changed their traffic management structure and it is mistaking your encrypted SSH connection as an encrypted P2P connection. If you can turn off the web server at the other end temporarily without causing to much trouble, try running SSH on port 80 to see if it runs better that way (if it does then something somewhere is probably doing port based traffic shaping that is affecting SSH). You could also try a bulk transfer via SSH (i.e. a download via SCP/SFTP) running through a HTTP based tunnel like this one to see what difference it makes (it will need to be a bulk transfer, as otherwise any performance issues will be hidden by the extra latency added by the tunnel).