Thread:Extension talk:User Merge and Delete/Usernames with "@" character cannot be deleted or merged/reply

Since 1.15 or even earlier the @ character is used at to edit user rights on other wikis (see $wgUserrightsInterwikiDelimiter). It is also set as invalid character for usernames in $wgInvalidUsernameCharacters. Try unset( $wgInvalidUsernameCharacters['@'] );