I'll give a full background here, not sure how relevant some of it will be but it can't hurt...
So I had a VM running ubuntu server 13.10 that I recently upgraded 14.04 LTS.
After the upgrade, it appeared that I was unable to ssh into the machine. Login via the local console was fine, and ssh out was okay.
After some fiddling, I discovered that ssh only fails for one particular username. Using a second username proved successful.
I have tried multiple ssh clients and each time I receive the same error for that one user:
Software caused connection abort.
Now, from what I gather this comes from Windows itself, so I ask you, what possible reasons for this could there be?
PuTTY logs show the public key authentication succeeding, and then suddenly for no apparent reason, disconnecting:
Event Log: Access granted Outgoing packet #0x8, type 90 / 0x5a (SSH2_MSG_CHANNEL_OPEN) ... Outgoing raw data ... Event Log: Network error: Software caused connection abort
I've tried this with multiple ssh clients, from multiple Windows machines, across three different sites, and the result is always the same. One user works fine, the other fails, so that would suggest something wrong with the Ubuntu box no?
So, following Chirag64's suggestion,
ssh 127.0.0.1 connects fine, however
ssh <baduser>@127.0.0.1 fails with the same error message.
I've checked group memberships, authentication and user shell, which all match up with the working user.
su baduser allows me to change to that user, and make outgoing ssh connections however, an ssh connection to loopback using current user fails.
It just seems to be incoming connections to that one user for some reason or another.