Transmission Overheat - what can cause it?

Yesterday was a pretty hot day here in São Paulo Brazil and my 330i was behaving normally when the Transmission Overheat icon appeared. This is not the first time. The previous occurrence was an oil cooler failure confirmed when i saw oil mixed with coolant. This time, when it cooled down, I opened the coolant reservoir and it was oil free. So I guess we can eliminate oil cooler failure, right? So can it be?