I had my server online and got this exception, idk how to reproduce because I wasn't connected
09.10 02:49:38 [Server] INFO Failure message: Mapper Base (mapperbase) encountered an error during the common_setup event phase
09.10 02:49:38 [Server] INFO java.lang.ArrayIndexOutOfBoundsException: 512
09.10 02:49:38 [Server] INFO Mod Version: 1.16.5-2.4.0.0
09.10 02:49:38 [Server] INFO Mod Issue URL: https://github.com/MapperTV/mapperbase/issues
09.10 02:49:38 [Server] INFO Exception message: java.lang.ArrayIndexOutOfBoundsException: 512
09.10 02:49:38 [Server] INFO Stacktrace:
09.10 02:49:38 [Server] INFO at it.unimi.dsi.fastutil.objects.Object2IntOpenCustomHashMap.rehash(Object2IntOpenCustomHashMap.java:1154) ~[?:?] {re:classloading}
09.10 02:49:38 [Server] INFO at it.unimi.dsi.fastutil.objects.Object2IntOpenCustomHashMap.insert(Object2IntOpenCustomHashMap.java:305) ~[?:?] {re:classloading}
09.10 02:49:38 [Server] INFO at it.unimi.dsi.fastutil.objects.Object2IntOpenCustomHashMap.put(Object2IntOpenCustomHashMap.java:313) ~[?:?] {re:classloading}
09.10 02:49:38 [Server] INFO at net.minecraft.util.registry.SimpleRegistry.func_243537_a(SimpleRegistry.java:78) ~[?:?] {re:classloading,pl:runtimedistcleaner:A}
09.10 02:49:38 [Server] INFO at net.minecraft.util.registry.SimpleRegistry.func_218382_a(SimpleRegistry.java:70) ~[?:?] {re:classloading,pl:runtimedistcleaner:A}
09.10 02:49:38 [Server] INFO at net.minecraft.util.registry.SimpleRegistry.func_218381_a(SimpleRegistry.java:100) ~[?:?] {re:classloading,pl:runtimedistcleaner:A}
09.10 02:49:38 [Server] INFO at net.minecraft.util.registry.Registry.func_218322_a(Registry.java:456) ~[?:?] {re:classloading,pl:runtimedistcleaner:A}
09.10 02:49:38 [Server] INFO at tv.mapper.mapperbase.world.BaseFeatures.registerFeatures(BaseFeatures.java:27) ~[mapperbase:1.16.5-2.4.0.0] {re:classloading}
09.10 02:49:38 [Server] INFO at tv.mapper.mapperbase.MapperBase.setup(MapperBase.java:56) ~[mapperbase:1.16.5-2.4.0.0] {re:classloading}