Psiphon VPN software server addresses are getting filtered on some of Russian ISPs (the ones that block Tor). The filtering is done with DPI, by blocking the TCP session after some amount of transmitted packets.
The filtering applies to custom Psiphon’s HTTP protocol, custom DNS masking protocol, and what’s more importantly, to SSH (a regular one) protocol.
ssh firstname.lastname@example.org — is one of Psiphon IPs. The connection is “hung” after several packets transferred. SSH client receives the server keys but never manages to establish connection to the password entering stage — the packets are filtered and dropped.
The filtering applies only towards known Psiphon IP addresses.