Playing in a single server world. A few minutes after playing, when trying to open the backpack, I get disconnected from the internal server.

The 'latest' log seems to be confirming Inmis as the cause:

[12:39:13] [Netty Server IO #5/ERROR]: Encountered exception while handling in channel with name "inmis:open_backpack"
java.util.ConcurrentModificationException: null
at java.util.HashMap$HashIterator.nextNode(HashMap.java:1597) ~[?:?]
at java.util.HashMap$KeyIterator.next(HashMap.java:1620) ~[?:?]
at net.minecraft.class_4558.method_22510(class_4558.java:60) ~[client-intermediary.jar:?]
at net.minecraft.class_2066.method_24362(class_2066.java:55) ~[client-intermediary.jar:?]
at net.minecraft.class_2066.method_8950(class_2066.java:51) ~[client-intermediary.jar:?]
at net.minecraft.class_3222$2.method_7635(class_3222.java:235) ~[client-intermediary.jar:?]
at net.minecraft.class_1703.method_34246(class_1703.java:218) ~[client-intermediary.jar:?]
at net.minecraft.class_1703.method_7623(class_1703.java:175) ~[client-intermediary.jar:?]
at net.minecraft.class_1703.method_7596(class_1703.java:138) ~[client-intermediary.jar:?]
at net.minecraft.class_3222.method_14235(class_3222.java:400) ~[client-intermediary.jar:?]
at net.minecraft.class_3222.method_17355(class_3222.java:1039) ~[client-intermediary.jar:?]
at draylar.inmis.item.BackpackItem.openScreen(BackpackItem.java:58) ~[inmis-2.5.2-1.18.2.jar:?]
at draylar.inmis.network.ServerNetworking.receiveOpenBackpackPacket(ServerNetworking.java:45) ~[inmis-2.5.2-1.18.2.jar:?]
at net.fabricmc.fabric.impl.networking.server.ServerPlayNetworkAddon.receive(ServerPlayNetworkAddon.java:89) ~[fabric-networking-api-v1-1.0.21+d882b91560-a8c4d1390f5e44a7.jar:?]
at net.fabricmc.fabric.impl.networking.server.ServerPlayNetworkAddon.receive(ServerPlayNetworkAddon.java:38) ~[fabric-networking-api-v1-1.0.21+d882b91560-a8c4d1390f5e44a7.jar:?]
at net.fabricmc.fabric.impl.networking.AbstractChanneledNetworkAddon.handle(AbstractChanneledNetworkAddon.java:100) [fabric-networking-api-v1-1.0.21+d882b91560-a8c4d1390f5e44a7.jar:?]
at net.fabricmc.fabric.impl.networking.server.ServerPlayNetworkAddon.handle(ServerPlayNetworkAddon.java:84) [fabric-networking-api-v1-1.0.21+d882b91560-a8c4d1390f5e44a7.jar:?]
at net.minecraft.class_3244.handler$cim000$handleCustomPayloadReceivedAsync(class_3244.java:1714) [client-intermediary.jar:?]
at net.minecraft.class_3244.method_12075(class_3244.java) [client-intermediary.jar:?]
at net.minecraft.class_2817.method_12199(class_2817.java:38) [client-intermediary.jar:?]
at net.minecraft.class_2817.method_11054(class_2817.java:7) [client-intermediary.jar:?]
at net.minecraft.class_2535.method_10759(class_2535.java:172) [client-intermediary.jar:?]
at net.minecraft.class_2535.method_10770(class_2535.java:157) [client-intermediary.jar:?]
at net.minecraft.class_2535.channelRead0(class_2535.java:55) [client-intermediary.jar:?]
at io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:99) [netty-all-4.1.68.Final.jar:4.1.68.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) [netty-all-4.1.68.Final.jar:4.1.68.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) [netty-all-4.1.68.Final.jar:4.1.68.Final]
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357) [netty-all-4.1.68.Final.jar:4.1.68.Final]
at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1410) [netty-all-4.1.68.Final.jar:4.1.68.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) [netty-all-4.1.68.Final.jar:4.1.68.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) [netty-all-4.1.68.Final.jar:4.1.68.Final]
at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:919) [netty-all-4.1.68.Final.jar:4.1.68.Final]
at io.netty.channel.local.LocalChannel.readInbound(LocalChannel.java:299) [netty-all-4.1.68.Final.jar:4.1.68.Final]
at io.netty.channel.local.LocalChannel.finishPeerRead0(LocalChannel.java:445) [netty-all-4.1.68.Final.jar:4.1.68.Final]
at io.netty.channel.local.LocalChannel.access$400(LocalChannel.java:50) [netty-all-4.1.68.Final.jar:4.1.68.Final]
at io.netty.channel.local.LocalChannel$5.run(LocalChannel.java:403) [netty-all-4.1.68.Final.jar:4.1.68.Final]
at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:164) [netty-all-4.1.68.Final.jar:4.1.68.Final]
at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:469) [netty-all-4.1.68.Final.jar:4.1.68.Final]
at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:500) [netty-all-4.1.68.Final.jar:4.1.68.Final]
at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:986) [netty-all-4.1.68.Final.jar:4.1.68.Final]
at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74) [netty-all-4.1.68.Final.jar:4.1.68.Final]

Full log: https://paste.gg/p/anonymous/6a39eb306f24423eae2063b9aa87f609

Happened three times already now, within a relatively short timeframe.

Strangely enough, it worked fine before. I played yesterday and did not notice any problems like that, so I'm not quite sure what happened. The only mod I added since then is Incendium_v5.0.3, a nether generation mod. I don't see how this one could cause any issues?

It also doesn't happen every time I open the backpack. The first few times accessing the backpack works just fine.

Played around with it some more. Opening the backpack from the hotbar does not cause any issues. Opening the equipped backpack via hotkey causes the crash, without fail after opening the backpack for the third or fourth time that way (within seconds if I do it right after re-opening the world). Changed the hotkey, same issue still.

I tried adding some thread safety to the class that's breaking in https://www.curseforge.com/minecraft/mc-mods/quality-of-life-qol-fixes, seems to have fixed it.

Wanted to make the change in inmis, except the ShulkerBoxTooltips repo can't load for me

Same here with my modpack: https://www.curseforge.com/minecraft/modpacks/spoornpack-fabric

That seems to have fixed it for me, thank you so much!

I was having trouble reproducing this issue, but then I noticed all my Backpack opening logic was running on the network thread. 🤦‍♀️

Haven't been able to confirm this actually fixes the issue (because I can't trigger it), but there's no way this wasn't the problem... please let me know if you run into the issue again. :)