[15:12:26] [Netty Local Client IO #2/ERROR] [FML]: There was a critical exception handling a packet on channel ffs
java.util.ConcurrentModificationException: null
at java.util.WeakHashMap$HashIterator.nextEntry(Unknown Source) ~[?:1.8.0_381]
at java.util.WeakHashMap$ValueIterator.next(Unknown Source) ~[?:1.8.0_381]
at java.util.AbstractCollection.removeAll(Unknown Source) ~[?:1.8.0_381]
at com.lordmau5.ffs.util.TankManager.remove(TankManager.java:88) ~[TankManager.class:?]
at com.lordmau5.ffs.network.handlers.client.OnTankBreak.channelRead0(OnTankBreak.java:15) ~[OnTankBreak.class:?]
at com.lordmau5.ffs.network.handlers.client.OnTankBreak.channelRead0(OnTankBreak.java:11) ~[OnTankBreak.class:?]
at io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:105) ~[SimpleChannelInboundHandler.class:4.1.9.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362) ~[AbstractChannelHandlerContext.class:4.1.9.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348) ~[AbstractChannelHandlerContext.class:4.1.9.Final]
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340) ~[AbstractChannelHandlerContext.class:4.1.9.Final]
at io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:102) ~[MessageToMessageDecoder.class:4.1.9.Final]
at io.netty.handler.codec.MessageToMessageCodec.channelRead(MessageToMessageCodec.java:111) ~[MessageToMessageCodec.class:4.1.9.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362) ~[AbstractChannelHandlerContext.class:4.1.9.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348) ~[AbstractChannelHandlerContext.class:4.1.9.Final]
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340) ~[AbstractChannelHandlerContext.class:4.1.9.Final]
at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1334) ~[DefaultChannelPipeline$HeadContext.class:4.1.9.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362) ~[AbstractChannelHandlerContext.class:4.1.9.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348) ~[AbstractChannelHandlerContext.class:4.1.9.Final]
at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:926) ~[DefaultChannelPipeline.class:4.1.9.Final]
at io.netty.channel.embedded.EmbeddedChannel.writeInbound(EmbeddedChannel.java:274) ~[EmbeddedChannel.class:4.1.9.Final]
at net.minecraftforge.fml.common.network.internal.FMLProxyPacket.processPacket(FMLProxyPacket.java:99) [FMLProxyPacket.class:?]
at net.minecraft.network.NetworkManager.channelRead0(NetworkManager.java:147) [gw.class:?]
at net.minecraft.network.NetworkManager.channelRead0(NetworkManager.java:49) [gw.class:?]
at io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:105) [SimpleChannelInboundHandler.class:4.1.9.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362) [AbstractChannelHandlerContext.class:4.1.9.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348) [AbstractChannelHandlerContext.class:4.1.9.Final]
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340) [AbstractChannelHandlerContext.class:4.1.9.Final]
at ic2.core.network.RpcHandler.channelRead0(RpcHandler.java:123) [RpcHandler.class:?]
at ic2.core.network.RpcHandler.channelRead0(RpcHandler.java:25) [RpcHandler.class:?]
at io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:105) [SimpleChannelInboundHandler.class:4.1.9.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362) [AbstractChannelHandlerContext.class:4.1.9.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348) [AbstractChannelHandlerContext.class:4.1.9.Final]
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340) [AbstractChannelHandlerContext.class:4.1.9.Final]
at net.minecraftforge.fml.common.network.handshake.NetworkDispatcher.handleClientSideCustomPacket(NetworkDispatcher.java:382) [NetworkDispatcher.class:?]
at net.minecraftforge.fml.common.network.handshake.NetworkDispatcher.channelRead0(NetworkDispatcher.java:268) [NetworkDispatcher.class:?]
at net.minecraftforge.fml.common.network.handshake.NetworkDispatcher.channelRead0(NetworkDispatcher.java:72) [NetworkDispatcher.class:?]
at io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:105) [SimpleChannelInboundHandler.class:4.1.9.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362) [AbstractChannelHandlerContext.class:4.1.9.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348) [AbstractChannelHandlerContext.class:4.1.9.Final]
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340) [AbstractChannelHandlerContext.class:4.1.9.Final]
at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1334) [DefaultChannelPipeline$HeadContext.class:4.1.9.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362) [AbstractChannelHandlerContext.class:4.1.9.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348) [AbstractChannelHandlerContext.class:4.1.9.Final]
at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:926) [DefaultChannelPipeline.class:4.1.9.Final]
at io.netty.channel.local.LocalChannel.finishPeerRead0(LocalChannel.java:443) [LocalChannel.class:4.1.9.Final]
at io.netty.channel.local.LocalChannel.access$500(LocalChannel.java:49) [LocalChannel.class:4.1.9.Final]
at io.netty.channel.local.LocalChannel$5.run(LocalChannel.java:397) [LocalChannel$5.class:4.1.9.Final]
at io.netty.channel.DefaultEventLoop.run(DefaultEventLoop.java:54) [DefaultEventLoop.class:4.1.9.Final]
at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:858) [SingleThreadEventExecutor$5.class:4.1.9.Final]
at java.lang.Thread.run(Unknown Source) [?:1.8.0_381]
[15:12:26] [Netty Local Client IO #2/ERROR] [FML]: Network Disconnect: A fatal error has occurred, this connection is terminated
[15:12:26] [Netty Local Client IO #2/INFO] [EnderCore]: Reset configs to client values for com.enderio.core.common.config.ConfigHandler
[15:12:26] [Netty Local Client IO #2/INFO] [FluxNetworks]: FluxNetworkData has been unloaded
[15:12:26] [Netty Local Client IO #2/INFO] [mcjtylib_ng]: Client disconnected from server
[15:12:26] [Netty Local Client IO #2/INFO] [mcjtylib_ng]: Cleaning up RFTools dimensions
[15:12:26] [Netty Local Client IO #2/INFO] [Astral Sorcery]: Cleaning client cache...
[15:12:26] [Netty Local Client IO #2/INFO] [Astral Sorcery]: Cleared cached client data! Disconnected from server.
[15:12:26] [Netty Local Client IO #2/INFO] [scannable]: Building block state lookup table...
[15:12:26] [Client thread/INFO] [HammerCore]: Cleaning 0 textures from VRAM
[15:12:26] [Client thread/INFO] [HammerCore]: Interrupting 0 image download threads
[15:12:26] [Server thread/INFO] [FluxNetworks]: FluxNetworkData has been successfully loaded

As the CurseForge page mentions:

Support for any version that isn't 1.20.1 has been discontinued.