Yeah it's my english.
What I meant was "to set" in past tense... which I now know is "set" even in past tense...
So, to come back to what I said, I have added a folder (the folder which contains all my music) to the watched folders of Play.
Everything was going fine and the "watch folders" feature was working correctly. Now, something has changed in my music folder that makes Play to crash when it scans my files at startup (the watch folder process).
This did not begin to happen when using a new version of Play, it happened I suppose when I added some new files in my music folder.
The source of the problem is probably a/some file(s) that Play doesn't like for some reason, but what are those files and why it makes Play crash, I don't know.