MC 1.16.5. Forge 36.1.0(3). 262 mods +- Kotlin. All default settings.
Without Kotlin, I create a world and go to it as many times as I want. With Kotlin, I create a world and can't log in later times.
Bad log with Kotlin (I can't log in to world a second time) - https://gist.github.com/kellixon/eac7f1f30970e1803acda3c0f1834143
Good log without Kotlin - https://gist.github.com/kellixon/ea8dd25060e8a951af13531521e5767b
My localpack (zip, only mods) - https://yadi.sk/d/ShAh2B5-_AfBNQ

Nobody's testing this and it's probably an issue with another mod that depends on KFF, so I'm going to close the issue

I'm getting a different error than you, but I can't test any further because my computer can't handle the modpack

What mods depend on Kotlin in your modpack?

Bountiful, but I temporarily removed it, only with Kotlin

It looks like the bad log doesn't have Kotlin but the good log does have Kotlin?

Seems like a structure error with one of the mods

Yes, I mixed up the logs in places. And I understand there is some incompatibility, or just some other mod has the wrong structure of something.

Yes, it would be great, but only if you are not difficult and have time!
ps. I allocate 12Gb of RAM for the build.