tl;dr: bad ssh config - check to see if you set ChrootDirectory in your global configuration settings and remove it
I spent a good 45mins on this problem and had to figure it out myself, so I figured I'd share with the world.
I was trying to ssh in to my home server and kept getting:
client_loop: send disconnect: Broken pipe
Looking this up online yielded no useful results - the most common reason that you encounter this message is that ssh killed the connection after a certain timeout. That was not my problem. I was getting this message immediately after connecting, and journalctl also gave me nothing useful, even with verbose or debug logging enabled.
It turns out that I had a bad configuration from setting up an sftp site. My ssh_config file looked something like this:
ChrootDirectory %h
Subsystem sftp internal-sftp
Match User sftp_user
ChrootDirectory %h/uploads/
ForceCommand internal-sftp
AllowTcpForwarding no
This initial ChrootDirectory %h was leftover from some previous editing. I was able to connect to my sftp site (but not regular ssh sessions) because the sftp_user settings were overriding the original ChrootDirectory.
The clue came when I tried to ssh in to root instead. I got:
Linux [My Hostname Here] #1 SMP PREEMPT_DYNAMIC [OS info here]
/bin/bash: No such file or directory
Connection to [hostname] closed.
/bin/bash definitely existed. That's when it clicked that ssh couldn't find /bin/bash because I had chroot'd to my home directory. I'm not sure why root gave me this message, while my regular user just gave me broken pipe.
The solution was to remove that global-level ChrootDirectory line, but keep the sftp settings. After that, I was able to ssh in just fine.
Hope this helps someone else with the same issue as me in the future