Allright. I looked in the Scumm Vm Forums and I found that this problem is due to the spanish version of the game (it's a fan translation or something like that), which is the version I was playing. There's a specific patch for this problem. I post here the link for the download:
I copied the file in the game directory, overwriting the original, removed the game from ScummVm, added it again, and then it worked perfectly. Hope this would help other people if they run into the same bug I did.
Thank you very much.