One day I was SSHing into my ArchLinux, but it didn't succeed. The thing didn't even attempt authentication. It said:
Key exchange failed.
No compatible cipher. The server supports these ciphers: AES-128-CTR,AES-192-CTR,AES-256-CTR,firstname.lastname@example.org,email@example.com,firstname.lastname@example.org
That was surprising. It did work earlier. Ok, Arch is one of those bleeding-edge distros. It does use newest of the new stuff in it. My client is SecureCRT and it has been serving me well for years, actually over decade. I had to confirm the connectivity with Cygwin's OpenSSH client. It worked just fine. Connection opens, no grievance from there. So, something must be wrong with my SecureCRT's settings. This is what the cipher list looked like in Session Options -> Connection->SSH2->Advanced:
Darn! It didn't have the newest big guns enabled. I must have ran too many upgrades to it. Apparently the upgrade doesn't enable that in my settings. I manually changed it into:
... which made the connection succeed.
I checked the server version number and it was OpenSSH_6.7p1. The sshd_config manual says:
Yes. I have tested that and didn't like it. I found no reason to change from my weapon-of-choice SecureCRT / SecureFX.
If you actually would have read my post, you would see, that this is a description of how I solved the connection problem, not a complaint about thing that won't work. My intention is to share solutions to people having similar issues than I encounter.