The other mascots might be corrupted by something. Rambley did state that his friends are not acting normal, and in the game we can see that Mollie and Lloyd looked unlike their cartoonish counterparts. Lloyd and Mollie's eyes are hollow and they lack their cartoonish elements, looking rather animalistic. In short, they looked corrupted. So something must have corrupted the mascots.
Remember that mini-game? In that game, Salem used some sort potion to corrupt Mollie, and called her "New and improved". The method of corruption could go in these ways:
The corruption has something to do with Salem. Maybe Salem is created to corrupt the mascots, or they represent the entity or something that corrupts the park. Since they are the arch nemesis of Rambley, they might be the one who did this out of spite. And they might be the one who is making Rambley's system glitchy because in the mini-game, once you beat the corrupted Mollie, the game glitches. It might be due to Salem's anger towards Rambley.
The Indigo Park Company wanted to revamp the mascots, hence Salem calling Mollie "New and improved" after the potion. The company accidentally corrupted them by creating some sort of substance. We have seen 3 mascots who are 3D so far and they all seemed corrupted. Maybe all the mascots got turned 3D and got corrupted (including the 3D version of Rambley). But the virtual assistant version of him is unaffected due to him being in another dimension.
Maybe, the employees stopped coming in because they are killed by the mascots. You can see there's blood on Mollie's beak and all over Lloyd's claws and body, so they must have killed someone. However, another reason for the employees to stop coming might be because of the experiment I mentioned above, or they have been replaced by the ai in game. Rambley pretty much knows everything about the park except what made it close down.
Anyway, something must have corrupted the park, and what Salem did might represent what happened to the mascots.