I don't really know if it's the correct way to fix it, but so far it seems to have worked.
My old posts are back to normal again and

1. In /etc/apache2/sites-available/ I edited my .conf file to contain the following line:

AddDefaultCharset utf-8

2. add this line to the mt-config.cgi:
PublishCharset utf-8

Concerning the Apache change you might wanna look here
Here's the important part:
AddDefaultCharset should only be used when all of the text resources to which it applies are known to be in that character encoding and it is too inconvenient to label their charset individually. One such example is to add the charset parameter to resources containing generated content, such as legacy CGI scripts, that might be vulnerable to cross-site scripting attacks due to user-provided data being included in the output. Note, however, that a better solution is to just fix (or delete) those scripts, since setting a default charset does not protect users that have enabled the "auto-detect character encoding"; feature on their browser.