What's causing this ERROR 2000 (HY000): Unknown MySQL error
Well I figured it out, and it isn't pretty. I guess I'll have to file a bug with MySQL.


The server is compiled with OpenSSL.
The working client is compiled with OpenSSL.
The client that can't connect is compiled with yaSSL.


The user I was trying to connect with used the sha256_password authentication plugin. When I dropped and re-created the user with mysql_native_password instead, the problem went away and I can connect from all clients. It appears that the sha256_password plugin doesn't work if the server and client are compiled against different SSL providers. (It might not even work with yaSSL at all.)