When trying to use serialization is bad as a java agent on a Mohist server with any version starting at 1.18.2 the server crashes with the message java.lang.reflect.InaccessibleObjectException: Unable to make field static final java.lang.invoke.MethodHandles$Lookup java.lang.invoke.MethodHandles$Lookup.IMPL_LOOKUP accessible: module java.base does not "opens java.lang.invoke" to unnamed module @6591f517.

Here's a log from a Mohist 1.20.1 server with this issue: https://mclo.gs/o4nssB6

Which command are you using to start the server?

java -javaagent:/aternos/other/serializationisbad.jar -jar server.jar

I'm not really sure what's causing this issue at the moment, but I saw similar behavior when using MultiMC.

It appears to be some incompatibility with wrappers around the Forge ModLauncher (MultiMC uses ForgeWrapper and Mohist itself also wraps ModLauncher).
But (at least to my knowledge) I'm not messing with any internals of ML that could cause this issue.

I will take a look at that again, maybe I overlooked something.
(Or maybe someone reading this has an idea?)

In the meantime you could just try to run SiB as a coremod instead of as an agent.

Could you please try adding --add-opens java.base/java.lang.invoke=cpw.mods.securejarhandler to the launch arguments before the -jar argument?

It says WARNING: Unknown module: cpw.mods.securejarhandler specified to --add-opens with the same error following it: https://mclo.gs/soEugHB

This might also be a problem with Mohist