The last thing I did in game? I was making 10 wool clothes for the quest (it takes 10 hours to make them).
A little while ago I've backed up and I've tried to reproduce the bug 15 times, without success. I mean, I put the wool on the loom just before midnight, then after 1 hour I tried to collect it. I did it 15 times and there were no problems. My guess is that this bug shows up after a few hours of play (as I did the last time). So my solution is that from now on it is better for me if I close the game every half hour (and then I reopen the saved game to go on).