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Looks like the mod has been patched already if you want to go the secure way. Either update it or change a config line manually. Closed issue on github with what to do: https://github.com/dogboy21/serializationisbad/issues/83

Same exact error. Just played last night without no issues. Seems like SerializationIsBad (mod that fixes vulnerabilities in other mods afaik) fails to grab the latest config from their servers due to a certificate error, so it might be a problem on their end. Might be worth bringing it up on their Github/Discord, they seem to be security oriented folk, so might be very responsive. Alternatively, since the mod does not seem to add any content, you could temporarily remove it from the pack and continue playing, but you will have vulnerabilities in unpatched old mods that SIB is fixing. If you get errors about missing content before loading the world, back out and make a backup of your saves before diving further.