I wrote this with the intention of sending it to Dreamhost Support, but found the solution by the end.

Before today, I had not logged in as thunderrabbit for a pretty
long time (over a year).

Earlier today, I reset thunderrabbit's password via
https://panel.dreamhost.com/index.cgi?tree=users

After a few minutes, via ssh, I logged into Canyonville using
thunderrabbit with the password

Logged in as thunderrabbit, I did not do much.  I just changed the
permissions of directory /home/thunderrabbit/robnugen.com from 700
to 755, so newly created domain temp.robnugen.com could serve the
(old) content at that URL.

After a couple hour break, I could not log in again as
thunderrabbit on Canyonville, even after carefully copy-pasting
the password.

I even tried resetting the password again, have waited several
minutes, including taking a shower and now typing this request..

Why can't I log in as thunderrabbit on Canyonville?

Related info:

In ~/.ssh/config with the following block

Host dreamhost
    HostName canyonville.dreamhost.com
    User thunderrabbit

At my command line $ prompt, I type

$ ssh dreamhost

and it replies 

Received disconnect from 64.90.48.21 port 22:2: Too many authentication failures
Disconnected from 64.90.48.21 port 22

If I add `IdentitiesOnly` to the block

Host dreamhost
    HostName canyonville.dreamhost.com
    User thunderrabbit
    IdentitiesOnly yes

$ ssh dreamhost
Host key fingerprint is SHA256:49CmYCDhMILX7yP5M/N0xDXUeimpNXRrRGUlTZ+o52U
thunderrabbit@canyonville.dreamhost.com's password: 
Permission denied, please try again.
thunderrabbit@canyonville.dreamhost.com's password: 
Permission denied, please try again.
thunderrabbit@canyonville.dreamhost.com's password: 
thunderrabbit@canyonville.dreamhost.com: Permission denied (publickey,password).

I AM able to log in to thunderrabbit via WebFTP at a URL like
https://files.dreamhost.com/#/c/64.90.48.21/thunderrabbit/REDACTED

For fun, after typing all the above, I am trying again to reset
the password.

While waiting for the new password reset, I will mention I have
tens of users on Canyonville, and all of them have been set up
with ssh keys.  thunderrabbit used to have an ssh key, but I lost
access to it when I changed computers.  This is just to say I am
99% I am doing things correctly, but just cannot understand why
the password authentication is not working when I log in via ssh..

Yeah it is still not working, and it has been 5 minutes since I
reset the password, so I hope you can figure it out.

OMG it works if I type the password and not use copy-paste buffer
(keyboard or mouse). ugh.