If the encoding is incorrect, it can cause problems, but it seems unlikely that it would cause that particular problem unless just those pagetitles have problem characters in them.
Repairing all tables in PhpMyAdmin is something to try in case the database itself is corrupted.
You can look at the core/config/config.inc.php file to see what encoding MODX expects, then you can look in the DB to see if the tables and the text fields inside the tables use that encoding.
There is more information on the character-set issue here along with a free script to correct things, but I haven't had a chance to check if the script works with the current version of MODX, so it may not.
http://bobsguides.com/convert-db-utf8.html
SiteCheck will check the character set of the DV, every table, and every text field for you and will check the validity of the resources to see if it can find any other problems with them, but it's not free.
http://bobsguides.com/sitecheck-promo.html