The error from the title pops-up when you enable a language you added a minute ago to the configuration (from the Settings -> Languages page). Usual case is you deal with an old database, that has collations, different from utf-8, and that messes up things. qTranslate FAQ will suggest you use a plugin called UTF Database Converter, but that one was last updated about two years ago the last time. And wiped out my content. That’s why I got a back-up ;)
What you’d better do (if you got WordPress 3.x) is get this one http://wordpress.org/extend/plugins/utf-8-db-converter/
It seems like an updated version of the previous one. The thing is that it does the business – just click a single button and it generates in several minutes a report on the results of the conversion. Do I need to say, that next thing is you check you’re website for issues with the content and plugins?
In the end you can go on with your qTranslate settings – add again the language of your choice and enable it. This time it’ll work like a charm!