There is no cookie prefix for one and it looks like you continually keep placing the wrong forum table prefix? is it really username_ I think not. It is most likely smf_
I can not tell you whats right or wrong here because this is not my site and you only know whats in your smf settings.php file.
Forums database table prefix for your forum (you may have chosen this initially when you installed your forum - leave blank if none or default):
smf_
Forum Cookie Prefix (if used, otherwise leave blank)
Some defaults include -
VB: bb, Threads: w3t_, phpBB2: phpbb2mysql
BLANK
SMF or WowBB Cookie Name (if used, otherwise leave blank)
Some defaults include - SMF: SmfCookie10,WowBB: wowbb
SMFCookie463 IS THAT CORRECT?
We do offer installation service.