I think ops ax is about right, probably Spring 2016 for the ST2"S"
I currently have an ST2 and my wife has one as well, both have been good bikes with no real issues. I have over 800 miles on my ST2, and over that time I have experienced just two errors (Sensor errors) that were both easily "cleared" by simply removing the battery for a few seconds and then replacing the battery - no sweat, probably momentary glitches. My wife's bike has about five hundred miles with no error messages at all - no problems whatsoever.
My big complaint is that in 2015 they did not offer any frame larger than 20 inch - now for 2016 they offer the 22 inch frame in bothe the ST2 and the new ST2S.......I wish my frame was the larger 22 inch flavor, but was not available in 2015 models. The other issue is the price, they are great bikes and probably one of the best e-bikes on the market, but the price is still ridiculous! The new ST2 "S" comes in just a hair under $10K ......WHOA!!!! Very hard to swallow that price!
The high price has me exploring other good ebikes, and I am currently looking at other brands like Haibike as an alternative.
If I could find a buyer for my current ST2 I might consider upgrading to the 2016 ST2 "S" ......I would need to add a rear rack for a tail bag ......the Stromer has many great advantages, and I'd like to stay with the brand to get a larger frame, but the price is daunting.