TinyMCE size bug

Hello, I’ve updated WordPress from 3.8.1 to 3.9.1 on a website I’m working on and migrated from qTranslate to mqTranslate 2.6.6.3.

Now, opening the Visual editor returns a badly sized textarea as you can see in this screenshot.

Any idea how could I fix this / why does this happens?

https://wordpress.org/plugins/mqtranslate/