ssh refuses to connect after upgrading to Big Sur

Attempting to ssh into my mini—which worked under 10.15 without fail—now either fails with a timeout or produces the ever so unhelpful error "kex_exchange_identification: Connection closed by remote host”:

ssh -vvvvA username@hostname (actual username and host elided for privacy)                                                                                                                                                                                

OpenSSH_8.1p1, LibreSSL 2.7.3

debug1: Reading configuration data /etc/ssh/ssh_config

debug1: /etc/ssh/ssh_config line 47: Applying options for *

debug1: Connecting to hostname port 22.

debug1: Connection established.

debug1: identity file /Users/username/.ssh/id_rsa type -1

debug1: identity file /Users/username/.ssh/id_rsa-cert type -1

debug1: identity file /Users/username/.ssh/id_dsa type -1

debug1: identity file /Users/username/.ssh/id_dsa-cert type -1

debug1: identity file /Users/username/.ssh/id_ecdsa type -1

debug1: identity file /Users/username/.ssh/id_ecdsa-cert type -1

debug1: identity file /Users/username/.ssh/id_ed25519 type 3

debug1: identity file /Users/username/.ssh/id_ed25519-cert type -1

debug1: identity file /Users/username/.ssh/id_xmss type -1

debug1: identity file /Users/username/.ssh/id_xmss-cert type -1

debug1: Local version string SSH-2.0-OpenSSH_8.1

kex_exchange_identification: Connection closed by remote host

Any ideas? Searching on the Web has been unhelpful.