The question "When are you in Berlin?" sounds, somehow, strange to me and informal. Is it OK to ask this way in a business environment or is there a better way?
What do you mean to ask?
When are you in Berlin?
This carries a little of the connotation that the person being questioned either has regularly scheduled visits to Berlin (response: "Every April and May") or has a longer itinerary already planned out, in which Berlin is a stop. (response: "After Holland and Zurich, but before Warsaw and Nagurskoye.")
If you are asking someone who is going to Berlin as a one off event, and not in the context of a longer trip, either
When will you be in Berlin? or
When are you going to Berlin? are more common. (AmE, West Coast)
When will you be in Berlin? is an alternative, but I see nothing wrong with When are you in Berlin? There is nothing informal about it.