Cookies are blocked or not supported error

I have 10 WordPress sites that I manage. All are on GoDaddy.com. Out of the 10, 3 give me the following error when I try and log on:

ERROR: Cookies are blocked or not supported by your browser. You must enable cookies to use WordPress.

If I key in my username and password again, I can get in.

Here is the URL of one that breaks: https://www.fvkg.com/dev/wp-login.php
Here is the URL of one that does not break: https://kevinmoriarity.com/wp-login.php

I looked at the test cookies (via Firefox) that WordPress submits and I noticed that “Host” for the sites that break show “www.domain.com” and the sites that do not break simply show “domain.com”, without the “www”. Where does this “host” value come from?

What I have checked:

1. My CNAME settings all look the same for the ones that work vs the ones that break.
2. My settings for WordPress Address and Site Address are the same in every installation and none have the “www”.
2. I looked in the MySQL database and the home and siteurl records in wp_options are all without the “www” – and both records are the same.
3. None of the 3 that break share a common plugin.
4. All my sites – the ones that work and the ones that break use the same theme: Suffusion.

Where is it getting this “host” value? Could that be what is causing this error? Any ideas?

Thank you.