Michael,
you mention "You need to have a page charset coherent with your database charset (hp8)"
Yes. I don’t know what is the "hp8" charset, but if you use the utf-8 as html charset (as I saw on your site), you logically should use an utf8 database charset.
Could you check what is the charset parameter defined in your modx configuration file, see manager/includes/config.inc.php
You have probably something like this:
$database_connection_charset = 'hp8';
is it true ?
Where would i find out if my page charset coherent with my database charset?
I don’t know which page charset should be coherent with "hp8" as database charset, but I am sure that with an utf-8 html page charset, you should have a database with an "utf8" database charset.
So, to resume:
1/ what is your database charset - use phpmyadmin for instance, to see your database charset.
Do you remenber during the MODx install, why you choose the "hp8" as database charset. Why do you use it ? (do you come from an another planet?

)
2/ which is the value of the $database_connection_charset in your config file ? Could you check it ?
3/ If "hp8" is not a mandatory charset for your database, I stronly recommend to redo a modx installaion with "utf8" as database charset