SSH error "Connection reset by peer" - no obvious cause

I'm trying to SSH a shared host server and everything has been working fine for over a year, but now without explanation whenever I try to connect to it I get:

ssh_exchange_identification: read: Connection reset by peer

Now I've read a few forum posts and questions on here but all none of the usual suggestions seem to work in this case. I think I've narrowed it down to my router because everything works from other networks (so my laptops and the server are both ruled out as the cause of the problem).

Also the problem occurs using both certificate and password authentication, I have Mac and Ubuntu laptops to try things on and the server I believe is some flavour of Linux.

I'd appreciate any help - I'm at a loss here and I feel like I must be missing something obvious... Thanks in advance.

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