What is causing sewer odor in the 2nd floor shower?

Hmmm.. not giving us anything to base an answer on here? How about telling us when the issue started? How about telling us when the odor is strongest... morning, evening, after a shower, when someone uses a toilet... that kind of stuff?

How many sinks in the bathroom? Is the shower used frequently or barely ever used? Is this the only or primary bathroom in the house/apartment or are there a number of other bathrooms? What floor is this bathroom on?

Have you tried to search out the issue in any way on your end. If so, what did you do?

Have you tried snaking the drain? How about removing the shower strainer and looking down into the drain pipe... probably some hair/soap/scum buildup there...remove if can visually confirm?

Finally, my first thought was that the shower could be "burping" sewer gasses into the home as a result of a blocked vent (shower or entire bathroom vent), but need information to help any more, OK? Answer my question the best you can...

Back to you...

Mark

Also, low water level in the trap / no trap...

I am with MASS and Milo, it could also be what is called the Bernoulli Effect. Wind blowing across your vent stack on the roof can suck/siphon water from a trap which will allow sewer gas in. You could also have a bad wax ring under your toilet.