At first, the server used to crash, because of a seemingly endless list of mobs of the same kind next to each other.
I deleted the mod, reinstalled it - now the list of mobs is empty, but the server still keeps crashing the same way [in the server terminal, on the left side there is an endless repeating row of numbers, and on the right side you have mods that contain mobs and their ids] - so I assume, it is trying to go through all the mods that have mobs available, but this inevitably overloads the server, and causes it to crash [Payload may not be larger than 1048576 bytes] and [System.OverflowException: VarInt too big]

Please share the crash logs if possible so that I can help you out.