Not sure, if this is something generally well known, but I´ll post it here, since it may help someone in the future:

after upgrading to 1.5.6c and installing CKEditor (used HTMLarea before all the time) I noticed that specific characters in my language (slovak, in this case) were transformed by CKEditor to HTML entities,

solution - I added this line:

---
config.entities_latin = false;
---

to the ckeditor file config.js (editors/ckeditor)

that solved the problem, all characters appear as they should