Lost ability to login; likely culprit "headers already sent error"

I am unable to log in to my wordpress site. I only found this out when I logged out and could not log back in. My partner is able to log in because his computer’s “Remember me” setting is on.

I went through the suggestions on the Login Trouble codex page and did not find a solution.

Here is what I know. My error log is giving two errors at the same time (actually, there are some other MySQL errors to, but they seem unrelated).

Error:
PHP Warning: Cannot modify header information – headers already sent by (output started at /home/santab/public_html/mbc-wp/wp-admin/admin-ajax.php:990)

I have gone in to that file. Here is the line it refers to:

if ( $do_lock && empty( $_POST['auto_draft'] ) && $id && is_numeric( $id ) ) {
		$lock_result = wp_set_post_lock( $id );
		$supplemental['active-post-lock'] = implode( ':', $lock_result );
	}

The other error that precedes this one indicates what is going wrong on line 990:
PHP Warning: implode() [<a href='function.implode'>function.implode</a>]: Invalid arguments passed in /home/santab/public_html/mbc-wp/wp-admin/admin-ajax.php on line 990

One other thing that is worth considering. The error log reports these errors as June 2, 2012. I upragded from 3.3.1 to 3.3.2 between then and now.

Thanks for any help.

Cheers