After a lot of reading, it seems to me that there's problem with the s2disk utility, and so I've worked around the problem using an older, more stable method of hibernation:
sudo echo disk > /sys/power/state
This will hibernate the machine, and resume the session when it is powered back up. It's not as fast as s2disk, and doesn't display any information while hibernating (it shows a black screen unless there are errors), but is far more stable, for my set up at least.
I put the command into a one line script, in /usr/bin/hib-script.sh, then added an entry in sudoers (see this post) to remove the requirement for a password to be entered. Finally, I added a launcher to Cairo-dock, so that now I have a consistent, stable hibernation function for my Linux Mint installation.