If you scroll down, you can see that it is repeatedly spamming "Skipping entity with ID" and nothing that follows, so I don't know what to do about it. Could any of you suggest anything? I also included the bootup logs there.
I'm not sure how mutant creatures is setup exactly, but from my research it seems that it does override some vanilla spawning. This being the case, I'd check the .cfg of the mod to see exactly how you have spawning setup. It seems like there is an argument about a mob spawning and whether it is allowed.
I'd experiment with /gamerule doMobSpawning false
This might resolve it. Its also possible that setting that to true would as well. If its like Mo Creatures in its generation, then sometimes mobs get "hung up." I doubt, however, this is causing a lot of lag...you seem to have server warnings before these pop up as well.
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