We use "such a/an" when refering to something equal or of the same kind of what was said before. And we use only "such" when we are referring particularly to that thing in question. With examples will be easier to understand the difference. Observe the two instances:
Using only "such" + {noun in question}
"The place was filled with gold. To enter such place one had to leave his pocktes and bags outside." Here, the author refers to that place, exclusively, and not any place alike.
Using "such a/an" + {noun in question}
"The place was filled with gold and guards. To enter such a place, one had to leave his clothes outside." Here, the author refers to 'any' place like that. "To enter such a place..." that means, that and any place of that kind, "one had to...".
That's the difference between "such" and "such a/an".
In the Asker's example, "in such an encouraging environment, we were able to cooperate and work on many projects together" means that they were able to cooperate in that specific environment and in any environment alike.
However, we don't know what's the text before the sample he provided. Perhaps only "such" could fit, too -- if it were referring exclusively to that specific environment.
The difference is more in the feeling of what exactly one wants to convey.