I had this same problem on a 740T and 940T. I've never experienced it on my 850, but when it happens, I'm prepared.

This problem was a result of a failing relay that allows the fuel injectors to deliver fuel to the cylinders. The motor cranks and cranks, but it never turns over. For me, the problem never was specific to heat or cold. It would happen at the most bizarre times. I'm sure that an error codes is probably stored in the computer....check it out.

FYI, this relay on the 740 and 940 is the same as the one that powers the self starting radiator fan. When in a pinch and if you know where to find them, they can be interchanged in case the car dies without warning. They are identical relays.

On the 850, there are two relays - side-by-side - that sit right above the radiator fan. The one in the center looks very much like the 740 and 940 model. I'll bet this is the problem. The ether is a coincidence and I don't think it has anything to do with your problem.