The monitor being off is for sure what causes the problem. If I closed the lid at this precise moment, the system would hibernate and then resume perfectly ok.
However, if I first ran the following command in a terminal to switch off the screen: $ sleep 1 && xset dpms force off and then closed the lid, the system would hibernate, but then would resume and switch off the screen. As I mentioned in my previous post, there is no way to switch on the screen. The only thing I can do is reboot the system. I've done plenty of tests, and this behaviour is always reproducible.
