Jackal (and later André) pointed out that the amount of Mercenaries spawning on this map gets too much to cope with. They spawn way too fast and the player cannot make progress due to hordes of them blocking the way. This was especially true for a player who just joined, when there would be literally 50+ blocking the player's path, depending how long the map had been running. I guess that in SinglePlay, the player is supposed to pick up the Invisibility item and use stealth to make progress, but in COOP this was impractical.
This was due to 2 Creature Factories with a capacity of 100,000 and a short interval between the next 2 in each factory that spawn, so it has been reduced to a much lower number now with a greater interval between the spawns, which is more suited for COOP play.
Thanks for pointing it out guys.
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WooF!