Vocabular Builder (Paperwhite2014) uses the wrong dictionary

[kindle_fan_2015]

Hi guys,

I got a bit annoyed with my (otherwise amazing) Kindle Paperwhite (2014) today. It has worked well for weeks, but today the Vocabulary Builder suddenly switched its dictionary language from French to English, thus being useless to me.
I already checked the Metadata of my french book using the Calibre tool and the language is set to French.
If I go to Settings > Device Options > Language and Dictionaries > Dictionaries, I got 2 entries listed, English and French and both are set to the correct dictionary that I prefer.
I also restarted the device, now I don't really know what I could try next in order to fix this problem.
Did anybody run into this problem before or is there a known fix?

Thanks so much,

Peter