Work in other European countries

Hi, i am in Netherlands on a Highly skilled migrant visa. I work for a consulting firm in Amsterdam, we do have customers in other European countries as well. So, i'll have to travel to other European countries to carryout consulting activities. Could you please guide me to a official statement whether is it legal to work in other European countries other than the host country?
Note: I'll travel back to the Netherlands for weekends and my salary is paid only in Netherlands.

The Vander Elst judgment.

The UK and Ireland require entry clearance for Vander Elst workers, but I don't think that applies within the Schengen Zone. But wait for others to respond.