Enable $debug in /conf/_advanced.php and look at the Debuglog at the bottom of the page.

Is it a "403 - Spam" error from b2evo?

Probably this is mod_security, which chokes on the "curl", which is a library/program often used to hack sites. If so, this has nothing to do with b2evo, but with your Apache setup.

I am getting "setcookie cookieb2evouser failed!" at the top...

Do you have a link to your blog?
Does any output occur before the "setcookie .. failed"?

No output occurs before that... it is the first thing that appears on the screen AFTER I hit the login button for the admin section.

http://jhsands.com/blog2/

I don't get the error now, but you have disabled debugging again.
Check your $baseurl and $cookie_* settings.

At least, when I manually add ?skin=custom to the URL, it sends me a cookie (and this should use the same settings). So, already fixed?

btw: next time please start a new topic for a new problem please.

I responded with the error it gave me when I enabled debug mode. I have checked the settings and everything appears to be set correctly.

Ok. Go into /blogs/b2evocore/_functions_users.php around line 80, where it should say:

                if( !setcookie( $cookie_user, $log, $cookie_expires, $cookie_path, $cookie_domain ) )
                        printf( T_('setcookie %s failed!'). '<br>', $cookie_user );
                if( !setcookie( $cookie_pass, $user_pass_md5, $cookie_expires, $cookie_path, $cookie_domain) )
                        printf( T_('setcookie %s failed!'). '<br>', $cookie_user );

Change it to:

                echo '<pre>'; var_dump($cookie_domain, $cookie_path); echo '</pre>';
                if( !setcookie( $cookie_user, $log, $cookie_expires, $cookie_path, $cookie_domain ) )
                        printf( T_('setcookie %s failed!'). '<br>', $cookie_user );
                if( !setcookie( $cookie_pass, $user_pass_md5, $cookie_expires, $cookie_path, $cookie_domain) )
                        printf( T_('setcookie %s failed!'). '<br>', $cookie_pass );

(add the first line and fix the last line).

Now, what does it say, if you try to login?

Any chance that you have strange characters in your login/user name?

Because the last line in the original file is misleading, we cannot be sure yet, if setting the password or username cookie fails. So please try it.