According to your testing, does the issue occur with ONLY Immersive Portals mod and Fabric API?

Yes

Describe the issue

as soon as I use the mod and use the following command /spawn in the chat, the error occurs

Immersive Portale (für Forge)
1.16.5

[13:33:29 ERROR]: Error encoding packet
java.lang.RuntimeException: Missing Dimension null
at com.qouteall.immersive_portals.dimension_sync.DimensionIdRecord.getIntId(DimensionIdRecord.java:39) ~[immersive_portals:0.16-mc1.16.5-forge]
at com.qouteall.immersive_portals.dimension_sync.DimId.writeWorldId(DimId.java:24) ~[immersive_portals:0.16-mc1.16.5-forge]
at net.minecraft.network.play.server.SPlayerPositionLookPacket.handler$zhg000$onWrite(SourceFile:530) ~[?:?]
at net.minecraft.network.play.server.SPlayerPositionLookPacket.func_148840_b(SourceFile:52) ~[?:?]
at net.minecraft.network.NettyPacketEncoder.encode(NettyPacketEncoder.java:40) ~[?:?]
at net.minecraft.network.NettyPacketEncoder.encode(NettyPacketEncoder.java:14) ~[?:?]
at io.netty.handler.codec.MessageToByteEncoder.write(MessageToByteEncoder.java:107) ~[server-1.16.5-20210115.111550-extra.jar:?]
at io.netty.channel.AbstractChannelHandlerContext.invokeWrite0(AbstractChannelHandlerContext.java:738) ~[server-1.16.5-20210115.111550-extra.jar:?]
at io.netty.channel.AbstractChannelHandlerContext.invokeWriteAndFlush(AbstractChannelHandlerContext.java:801) ~[server-1.16.5-20210115.111550-extra.jar:?]
at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:814) ~[server-1.16.5-20210115.111550-extra.jar:?]
at io.netty.channel.AbstractChannelHandlerContext.writeAndFlush(AbstractChannelHandlerContext.java:794) ~[server-1.16.5-20210115.111550-extra.jar:?]
at io.netty.channel.AbstractChannelHandlerContext.writeAndFlush(AbstractChannelHandlerContext.java:831) ~[server-1.16.5-20210115.111550-extra.jar:?]
at io.netty.channel.DefaultChannelPipeline.writeAndFlush(DefaultChannelPipeline.java:1071) ~[server-1.16.5-20210115.111550-extra.jar:?]
at io.netty.channel.AbstractChannel.writeAndFlush(AbstractChannel.java:300) ~[server-1.16.5-20210115.111550-extra.jar:?]
at net.minecraft.network.NetworkManager.lambda$sendPacket$5(NetworkManager.java:213) ~[?:?]
at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:163) ~[server-1.16.5-20210115.111550-extra.jar:?]
at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:404) ~[server-1.16.5-20210115.111550-extra.jar:?]
at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:465) ~[server-1.16.5-20210115.111550-extra.jar:?]
at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:884) ~[server-1.16.5-20210115.111550-extra.jar:?]
at java.lang.Thread.run(Thread.java:829) ~[?:?]
[13:33:29 INFO]: XXXXJOJOXXXX lost connection: Internal Exception: io.netty.handler.codec.EncoderException: java.lang.RuntimeException: Missing Dimension null
[13:33:29 INFO]: XXXXJOJOXXXX left the game

Forge 1.16.5 is no longer supported. Your error is caused by a mod dynamically adding a dimension.