Can't connect to PaperMC server

[Anonymous941]

On 1.21.1, enabling the Forgified Fabric API (even with no mods) gives the error Invalid payload REGISTER! and prints to the console:

[11:07:28 ERROR]: Couldn't register custom payload
java.lang.IllegalArgumentException: Channel must contain : separator (attempted to use )
        at org.bukkit.plugin.messaging.StandardMessenger.validateAndCorrectChannel(StandardMessenger.java:504) ~[paper-mojangapi-1.21.1-R0.1-SNAPSHOT.jar:?]
        at org.bukkit.craftbukkit.entity.CraftPlayer.addChannel(CraftPlayer.java:2600) ~[paper-1.21.1.jar:1.21.1-132-b48403b]
        at net.minecraft.server.network.ServerCommonPacketListenerImpl.handleCustomPayload(ServerCommonPacketListenerImpl.java:169) ~[paper-1.21.1.jar:1.21.1-132-b48403b]
        at net.minecraft.network.protocol.common.ServerboundCustomPayloadPacket.handle(ServerboundCustomPayloadPacket.java:26) ~[paper-1.21.1.jar:1.21.1-132-b48403b]
        at net.minecraft.network.protocol.common.ServerboundCustomPayloadPacket.handle(ServerboundCustomPayloadPacket.java:13) ~[paper-1.21.1.jar:1.21.1-132-b48403b]
        at net.minecraft.network.protocol.PacketUtils.lambda$ensureRunningOnSameThread$0(PacketUtils.java:56) ~[paper-1.21.1.jar:1.21.1-132-b48403b]
        at net.minecraft.server.TickTask.run(TickTask.java:18) ~[paper-1.21.1.jar:1.21.1-132-b48403b]
        at net.minecraft.util.thread.BlockableEventLoop.doRunTask(BlockableEventLoop.java:151) ~[paper-1.21.1.jar:1.21.1-132-b48403b]
        at net.minecraft.util.thread.ReentrantBlockableEventLoop.doRunTask(ReentrantBlockableEventLoop.java:24) ~[paper-1.21.1.jar:1.21.1-132-b48403b]
        at net.minecraft.server.MinecraftServer.doRunTask(MinecraftServer.java:1535) ~[paper-1.21.1.jar:1.21.1-132-b48403b]
        at net.minecraft.server.MinecraftServer.doRunTask(MinecraftServer.java:201) ~[paper-1.21.1.jar:1.21.1-132-b48403b]
        at net.minecraft.util.thread.BlockableEventLoop.pollTask(BlockableEventLoop.java:125) ~[paper-1.21.1.jar:1.21.1-132-b48403b]
        at net.minecraft.server.MinecraftServer.pollTaskInternal(MinecraftServer.java:1512) ~[paper-1.21.1.jar:1.21.1-132-b48403b]
        at net.minecraft.server.MinecraftServer.pollTask(MinecraftServer.java:1505) ~[paper-1.21.1.jar:1.21.1-132-b48403b]
        at net.minecraft.util.thread.BlockableEventLoop.managedBlock(BlockableEventLoop.java:135) ~[paper-1.21.1.jar:1.21.1-132-b48403b]
        at net.minecraft.server.MinecraftServer.managedBlock(MinecraftServer.java:1464) ~[paper-1.21.1.jar:1.21.1-132-b48403b]
        at net.minecraft.server.MinecraftServer.waitUntilNextTick(MinecraftServer.java:1471) ~[paper-1.21.1.jar:1.21.1-132-b48403b]
        at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:1316) ~[paper-1.21.1.jar:1.21.1-132-b48403b]
        at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:329) ~[paper-1.21.1.jar:1.21.1-132-b48403b]