Cannot use git over SSH on PC

Very lost here, could use some help.

I recently moved physical address, and after the move I simply can’t access my github repos over ssh anymore on my PC. A simple clone of a repo that I own over ssh

git clone git@github.com:david-janssen/kmonad -vvvvv

eventually hangs on:

debug1: Sending command: git-upload-pack 'david-janssen/kmonad

I’ve remade my ssh-keys, uploaded the new keys, reinstalled my whole system, nothing works. The ssh output of git seems to indicate that authentication all works fine as well:

debug1: Server accepts key: /home/david/.ssh/id_rsa RSA
SHA256:<EDITED OUT, JUST IN CASE>
Enter passphrase for key '/home/david/.ssh/id_rsa':
debug1: Authentication succeeded (publickey).
Authenticated to github.com ([140.82.118.4]:22).

Even weirder: all of this works just fine on my laptop. I’ve done a clean install of my system, disabled my firewall, my gitconfig is tiny (just my name and my email address)- and setting the ssh command to ssh -v for debug purposes).

I’ve tried googling the issue but haven’t turned up anything helpful. Super stuck, any suggestions?

Oh, I should probably also mentiond, this is under Linux (NixOS 20.03)

I’m not sure if that’s what it is, but I’ve seen similar issues with misguided firewall settings (including ISP-side!) that block the ICMP packets necessary for Path MTU Discovery. E.g. if some ISP router never tells the Github server that the packets it’s sending are too large to forward and just silently drops them, the server won’t know that it should resend the requested data in smaller packets. In that case (at least for TCP) limiting the TCP MSS can be a workaround.

What is a bit odd about this hypothesis is that things work on your laptop, which I assume is on the same network. Though theoretically the laptop might already be configured to limit the TCP MSS, or your router might apply different firewall settings for some reason. Or maybe a VPN is involved?

Tried it, but I’m afraid that limiting the TCP MSS didn’t work. Also: since cloning over HTTP does work, should that still be an issue? Additionally, the laptop is indeed on the same network, and there is no VPN involved.

Such a confusing issue.