Adding to the issue I had beetween my local and online installations with Yarpp which was due to a conflict with the Podcasting plugin, I had another issue.
Localy, the keywords inserted in the yarpp_keywords_cache table were fine, even with accentuated characters in the keywords.
Online, I realized, looking at the table, that keywords were cut just before an accentuated character, hence the keyword "vidéo" was stored as "v" ...
Tracking the outputs in the different functions of the yarpp plugin, I discovered that I had to replace line 11 of keywords.php :
$wordlist = mb_split('\s*\W+\s*', mb_strtolower($source))
$wordlist = mb_split('\s*\W+\s*', mb_strtolower($source , get_option('blog_charset')));
Hence specifying the encoding. Weird because I thought on the moment 'mb_regex_encoding(get_option('blog_charset'))' was doing the encoding stuff and it looks like it works on my local WP and not online.
I noticed my local installation on XAMPP is PHP 5.3 powered and online it's still 5.2.10.