First of all, thanks for your comments and suggestions.
I came back on Friday from my trip, and read this thread,
On Saturday morning I found an open guitar shop. (Just for you to know, Spain is closed on August
I bought a very simple humidifier, and put it within the sound box, and the case again to the case.
A couple of days later, it seems much better, but you can still hear some bad sound (some vibrations) when you pick hard on the strings.
I also bought a couple of string sets, with different gauges, to try.
But I haven't changed the strings which were put when the problem started.
I want to see how it evolves.
On the other hand, I feel my 'feeling' has gone away these days, no inspiration and really bad playing.
I hope to 'feel' better in a few days.