I have marked the problem in the SQL-Statement bold
SELECT c.msg_id, c.msg_author, c.msg_body, UNIX_TIMESTAMP(msg_date) AS msg_date, c.author_id, c.author_md5_id, c.msg_raw_ip, c.msg_hdr_ip, c.pid, u.
AS avatar_url FROM cpg1411_comments AS c LEFT JOIN `database name`.phpbb_users AS u ON c.author_id=u.user_id WHERE pid='1' ORDER BY c.msg_id ASC" on 0
It should be "u.user_avatar" instead of "u." only.
But if it is now working I fine, then let's forget that, thank you for testing
P.S.: Your site has a nice design, I like it