So-far-unused may be the problem. The trap may have dried out. Pour some water down the drain.

The funny part is that it seems to be moist...there are a couple of black flies.

"The funny part is that it seems to be moist...there are a couple of black flies."

Which doesn't mean there's enough water to fill the trap.

Is this what you're talking about?

http://www.homeconstructionimprovement.com/laundry-room-sewer-smell-solved/

Would you expect the problem would get solved by pouring a jug of water down the drain?

If not a dry trap. then sewage backwash in the trap. I have this happen every year mid-winter. I pour a pint of water with a little beach and the problem is solved.

Smelly basements can be tricky because sewer smells and mold smells are similar.

"Would you expect the problem would get solved by pouring a jug of water down the drain?"

Yes, if a dry trap is the cause. The 2nd graphic "In-Correct Washing Machine Hose Connection" is not relevant since there's as-yet no machine in place (note that "in-correct" is incorrect, LOL). The first graphic is relevant as the explanation if either a) there's no water in the trap; or b) there's not enough water to form a seal. Some of the water poured into the standpipe would pass through, the rest will settle in the trap.

I just put 5 litres of warm water down the drain. But I'm about to leave home for day so I'll find out if it did anything only late tomorrow night.

Did it work ? I have the same problem. I just moved into an apartment with an unused washer hook up. I will buy a washer later on ) I pired water down the drain and it helped but had some odor issues again the next day.

Yes, it worked. Of course, this is now many years later, and I have learned to run the water a few minutes every couple of weeks in unused sinks and tubs. I would suggest trying a few more rounds of very hot water.