Skyrim has A LOT of glitches. Restarting your computer won't help. This is a game issue, nothing else. Complete the quest and travel to another town, like Winterhold or something and save it there.
What he said. Run that quest without saving until you are out in nightcaller temple, travel elsewhere and you should be able to save. It's a bug in the Waking Nightmare(or whatever that daedric quest in nightcaller temple is called) when trying to save at certain points toward the end of the quest.