Thanks in advance for looking into this...

• Dynmap Version: v3.1-beta3a-409
• Server Version: paper 1.16.3 build 191
• Server Host (if applicable): Selfhosted on Debian Linux
• Steps to Replicate: Upgraded from paper-148.jar to paper-191.jar which automatically upgraded Minecraft 1.16.2 to 1.16.3, and now dynmap crashes at start (logs included below); all other plugs that we use are working without any problems

[X] I have looked at all other issues and this is not a duplicate
[X] I have been able to replicate this

Relevant portion of log file...

[09:09:06] [Server thread/INFO]: [dynmap] Enabling dynmap v3.1-beta3a-409*
[09:09:07] [Server thread/ERROR]: Error occurred while enabling dynmap v3.1-beta3a-409 (Is it up to date?)
java.lang.NoSuchMethodError: 'net.minecraft.server.v1_16_R2.IRegistryCustom net.minecraft.server.v1_16_R2.MinecraftServer.aX()'
<------>at org.dynmap.bukkit.helper.v116_2.BukkitVersionHelperSpigot116_2.getBiomeReg(BukkitVersionHelperSpigot116_2.java:70) ~[?:?]
<------>at org.dynmap.bukkit.helper.v116_2.BukkitVersionHelperSpigot116_2.getBiomeBaseList(BukkitVersionHelperSpigot116_2.java:84) ~[?:?]
<------>at org.dynmap.bukkit.DynmapPlugin.loadExtraBiomes(DynmapPlugin.java:781) ~[?:?]
<------>at org.dynmap.bukkit.DynmapPlugin.onEnable(DynmapPlugin.java:856) ~[?:?]
<------>at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:263) ~[patched_1.16.3.jar:git-Paper-191]
<------>at org.bukkit.plugin.java.JavaPluginLoader.enablePlugin(JavaPluginLoader.java:380) ~[patched_1.16.3.jar:git-Paper-191]
<------>at org.bukkit.plugin.SimplePluginManager.enablePlugin(SimplePluginManager.java:483) ~[patched_1.16.3.jar:git-Paper-191]
<------>at org.bukkit.craftbukkit.v1_16_R2.CraftServer.enablePlugin(CraftServer.java:499) ~[patched_1.16.3.jar:git-Paper-191]
<------>at org.bukkit.craftbukkit.v1_16_R2.CraftServer.enablePlugins(CraftServer.java:413) ~[patched_1.16.3.jar:git-Paper-191]
<------>at net.minecraft.server.v1_16_R2.MinecraftServer.loadWorld(MinecraftServer.java:468) ~[patched_1.16.3.jar:git-Paper-191]
<------>at net.minecraft.server.v1_16_R2.DedicatedServer.init(DedicatedServer.java:241) ~[patched_1.16.3.jar:git-Paper-191]
<------>at net.minecraft.server.v1_16_R2.MinecraftServer.w(MinecraftServer.java:939) ~[patched_1.16.3.jar:git-Paper-191]
<------>at net.minecraft.server.v1_16_R2.MinecraftServer.lambda$a$0(MinecraftServer.java:177) ~[patched_1.16.3.jar:git-Paper-191]
<------>at java.lang.Thread.run(Thread.java:834) [?:?]
[09:09:07] [Server thread/INFO]: [dynmap] Disabling dynmap v3.1-beta3a-409
[09:09:07] [Server thread/ERROR]: Error occurred while disabling dynmap v3.1-beta3a-409 (Is it up to date?)
java.lang.NullPointerException: null
<------>at org.dynmap.bukkit.DynmapPlugin.onDisable(DynmapPlugin.java:998) ~[?:?]
<------>at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:265) ~[patched_1.16.3.jar:git-Paper-191]
<------>at org.bukkit.plugin.java.JavaPluginLoader.disablePlugin(JavaPluginLoader.java:415) ~[patched_1.16.3.jar:git-Paper-191]
<------>at org.bukkit.plugin.SimplePluginManager.disablePlugin(SimplePluginManager.java:516) ~[patched_1.16.3.jar:git-Paper-191]
<------>at org.bukkit.plugin.java.JavaPluginLoader.enablePlugin(JavaPluginLoader.java:384) ~[patched_1.16.3.jar:git-Paper-191]
<------>at org.bukkit.plugin.SimplePluginManager.enablePlugin(SimplePluginManager.java:483) ~[patched_1.16.3.jar:git-Paper-191]
<------>at org.bukkit.craftbukkit.v1_16_R2.CraftServer.enablePlugin(CraftServer.java:499) ~[patched_1.16.3.jar:git-Paper-191]
<------>at org.bukkit.craftbukkit.v1_16_R2.CraftServer.enablePlugins(CraftServer.java:413) ~[patched_1.16.3.jar:git-Paper-191]
<------>at net.minecraft.server.v1_16_R2.MinecraftServer.loadWorld(MinecraftServer.java:468) ~[patched_1.16.3.jar:git-Paper-191]
<------>at net.minecraft.server.v1_16_R2.DedicatedServer.init(DedicatedServer.java:241) ~[patched_1.16.3.jar:git-Paper-191]
<------>at net.minecraft.server.v1_16_R2.MinecraftServer.w(MinecraftServer.java:939) ~[patched_1.16.3.jar:git-Paper-191]
<------>at net.minecraft.server.v1_16_R2.MinecraftServer.lambda$a$0(MinecraftServer.java:177) ~[patched_1.16.3.jar:git-Paper-191]
<------>at java.lang.Thread.run(Thread.java:834) [?:?]

This has already been fixed in Build #414, and build prior to 414 will not work on 1.16.3

Latest dev build can be found here