I am also having the same error* despite updating the Server.Properties file to 'level-type=biomesoplenty' and restarting the server. Any other options I can try?
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Internal Exception: java.lang.ClassCastException: net.minecraft.world.biome.Biome$Category cannot be cast to java.lang.String
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Fixed! I was using the incorrect Jar file. There are two that show up when dl the server folder, I was using minecraft.server.1.16.5.jar but when I tried forge-1.16.5-36.0.1.jar everything worked great. Silly mistake on my part, sorry!