I know there are other threads, but this is probably significant:
The save failure occurs at the exact instant that the Brimstone Vipers are triggered. I was using the Esc key to check if I could save files. The very instant the vipers appeared, the Save Game option was no longer available.
I immediately hit Quit while the animation was happening. Unlike previous times, the Save and Load options on the main menu were still available without reopening the game and a quick save had been created for the first round of combat. After combat, I could still save.
However, once I had talked to the commander in the fortress, save was disabled. Quit to the main menu did not fix the problem this time.