We're using SSH with public key authentication.
The user can connect from one machine, but not from another machine on a different network.
He was previously able to connect.
When we run ssh with -vvv we can see that our server is reached, but the ssh connection times out before he can do anything:
debug1: connect to address 188.8.131.52 port 30000: Operation timed out
(I've disguised our ip address above)
We turned on password authentication, and he can't connect with that either, from one machine,
We figure there is some sort of auto-generated blacklist, but don't know where to check.
EDIT: Well, magically, when we left it alone for a few minutes and tried again, the connection worked...