Migrated repo NAS between routers, now known hosts is always invalid

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I’ve migrated my NAS (repo via SFTP) between routers, thus it got a new IP, new local TLD. Kopia of course fussed about this

So I’ve tried to change the host.

No avail, the known host is invalid.

Okay, I’ve recreated the file with:

ssh -i ~/.ssh/id_ed25519_backup -oUserKnownHostsFile=~/.ssh/known_hosts.kopia $USER@$NAS.

The login worked, but Kopia didn’t accept the new entry. (At this point I was creating a new connection instead of modifying the old one.)

I’ve moved the old known hosts file to a new name, repeated the command, but it still does not work.

Tried the known hosts file contents: no change.

The error message I get each time:

Connect Error: INTERNAL: 
internal server error: 
unable to open SFTP storage: 
error establishing connecting: unable to dial [$NAS:22]:
&ssh.ClientConfig{
 Config:ssh.Config{
  Rand:io.Reader(nil), RekeyThreshold:0x0, KeyExchanges:[]string(nil), Ciphers:[]string(nil), MACs:[]string(nil)
},
User:"$USER",
Auth:[]ssh.AuthMethod{
 (ssh.publicKeyCallback)(0x18ab940)}, HostKeyCallback:(ssh.HostKeyCallback)(0x18a7e20), BannerCallback:(ssh.BannerCallback)(nil), ClientVersion:"", HostKeyAlgorithms:[]string(nil), Timeout:0
}: ssh: handshake failed: knownhosts: key mismatch

Edit: added newlines to the error message for readability.