Looking for a solution to my problem I came upon this thread: http://wordpress.org/support/topic/plugin-all-in-one-seo-pack-preg_replace-issue
It seems that the issue hasn't been resolved. I am using the latest version of All-in-One SEO Pack and when there are accented characters in the description, they are displayed as unknown charachers.
This results in getting the following message when trying to validate the page:
"Sorry, I am unable to validate this document because on line 32 it contained one or more bytes that I cannot interpret as utf-8 (...)"
The issue can be resolved temporarily by replacing line 533
$description = preg_replace("/\s\s+/", " ", $description);
$description = preg_replace("/\s\s+/u", " ", $description);