Bonjour à tous,
Thank you both for your answers ! The link mentioned by Chris has a very good explanation about the difference between 'ennuyeux' and 'ennuyant'. As Maarten pointed out though, 'ennuyeux' is mostly used nowadays (at least, in France - and as mentioned at the end of the article in BDL) rather than 'ennuyant'.
Merci de votre contribution et bonne journée !