ssh connection to gitlab fails behind home router, but https works most of the time

I am trying to connect to my repository on gitlab.com.

It used to work on my laptop both when I was at home and when I was at work. I was using ssh and had added the ssh keys to gitlab.

Suddenly some days ago it stopped working from home: I get:

$ git pull
Connection closed by 54.93.71.23
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

From within my work network, on the same laptop it works fine using ssh.

So now I switched to https and it works most of the time, but ocassionally it hangs and I get:

fatal: The remote end hung up unexpectedly
error: RPC failed; result=56, HTTP code = 0

From my work, this never happens, even if I try to commit the same changes just half an hour later.

What could be the problem with ssh? Am I behind a router firewall or did my provider change something?

Thanks!