[Server thread/ERROR] [net.minecraft.world.chunk.Chunk/]: A TileEntity type com.hollingsworth.arsnouveau.common.block.tile.PortalTile has thrown an exception trying to write state. It will not persist, Report this to the mod author
java.lang.NullPointerException: Cannot invoke "String.isEmpty()" because "p_229705_0_" is null
at net.minecraft.nbt.StringNBT.func_229705_a_(StringNBT.java:40) ~[?:?]
at net.minecraft.nbt.CompoundNBT.func_74778_a(CompoundNBT.java:150) ~[?:?]
at com.hollingsworth.arsnouveau.common.block.tile.PortalTile.func_189515_b(PortalTile.java:70) ~[ars_nouveau:1.24.16]
at net.minecraft.world.chunk.Chunk.func_223134_j(Chunk.java:489) ~[?:?]
at net.minecraft.world.chunk.storage.ChunkSerializer.func_222645_a(ChunkSerializer.java:291) ~[?:?]
at net.minecraft.world.server.ChunkManager.func_219229_a(ChunkManager.java:693) ~[?:?]
at java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:178) ~[?:?]
at java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:179) ~[?:?]
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:197) ~[?:?]
at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1625) ~[?:?]
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:484) ~[?:?]
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:474) ~[?:?]
at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:150) ~[?:?]
at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:173) ~[?:?]
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) ~[?:?]
at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:596) ~[?:?]
at net.minecraft.world.server.ChunkManager.func_219177_a(ChunkManager.java:341) ~[?:?]
at net.minecraft.world.server.ServerChunkProvider.func_217210_a(ServerChunkProvider.java:346) ~[?:?]
at net.minecraft.world.server.ServerWorld.func_217445_a(ServerWorld.java:937) ~[?:?]
at net.minecraft.server.MinecraftServer.func_213211_a(MinecraftServer.java:593) ~[?:?]
at net.minecraft.server.MinecraftServer.func_71260_j(MinecraftServer.java:651) ~[?:?]
at net.minecraft.server.dedicated.DedicatedServer.func_71260_j(DedicatedServer.java:626) ~[?:?]
at net.minecraft.server.MinecraftServer.func_240802_v_(MinecraftServer.java:864) ~[?:?]
at net.minecraft.server.MinecraftServer.func_240783_a_(MinecraftServer.java:269) ~[?:?]
at java.lang.Thread.run(Thread.java:831) [?:?]

Can you paste the full log with this? And are you sure the mods are the same version on the server and client?

Here is the full log.
https://gist.github.com/Timeken/cd3e7e58dc4e6a34f71aee76ec30e821

The mod version is the same on the server and client.

Would you be able to try this version to see if it fixes for you? May be needed on server and client.
https://maven.blamejared.com/com/hollingsworth/ars_nouveau/ars_nouveau-1.16.5/1.24.16.11/
If not, deleting the block via the command line should get you sorted until a release.

I put the updated version on both the client and the server and it works fine now. I did not have to delete the block either.