This is different from #215

Full log file:
crash-2020-11-22_23.04.26-server.txt

RER has already been removed from the server prior to this crash.

No it has not been removed.
at uk.me.desert_island.rer.WorldGenState.toNetwork(WorldGenState.java:144)
https://github.com/theorbtwo/RoughlyEnoughResources/blob/1.16/src/main/java/uk/me/desert_island/rer/WorldGenState.java

No it has not been removed.
at uk.me.desert_island.rer.WorldGenState.toNetwork(WorldGenState.java:144)
https://github.com/theorbtwo/RoughlyEnoughResources/blob/1.16/src/main/java/uk/me/desert_island/rer/WorldGenState.java

I'm new to modded. I thought removing RER from my server's mod list would be sufficient. Is it not?

You need to remove RER from server-setup-config.yaml.