After retracing my steps since the last autosave, I've basically pinpointed it to the following for reproduction: building a mine on the 'Factorium' ore. My final test was loading the good save, building then immediately destroying a vanilla mine on the patch, and then trying to save again. The 'Factorium ore' is added by `portal-research 0.7.9`, and appears to be a recolored version of uranium.Notice
Detected corruption of the game state. Game was not saved to prevent corruption of your save files. Please save the game into new file and send us the most recent working save and the corrupted save to forums.factorio.com. We will diagnose the problem and fix for future releases.
The save has a wreckage of a list of mods -- including a half-dozen or so that I ported from 0.14, and one or two unlabeled custom tweaks -- so I hope you can extract what you need without loading the file into a live game.
Also, note that while the game claims that it won't be able to load the corrupted save, I can load it just fine (well, it loads up and I can walk around; it very well might be broken and I just can't tell). Trying to save it again results in the same rejection -- even in the case where the loaded version doesn't have the mine.
EDIT: Update: Taking a broken save file, loading it with a version of the game without that mod, and then re-saving it appears to remove the "corruptedness".
EDIT2: Further investigation indicates that it is not, in fact, an issue of the mines, but one of the microwave transmitters. This was reported to the mod author by someone else already it turns out: https://mods.factorio.com/mods/docpaisl ... sion/16566. I still don't think a mod (even one with issues) should be corrupting game state though, so I'm leaving this up.
Many thanks for the speedy fix!