Minecraft: v1.21.1
Fabric Loader: v0.16.5
Fabric API: v0.105.0
Tom's Simple Storage version: v2.0.9

My server runs a Modpack but I also tested this on a test world using only Tom's Simple Storage and the crash still occurs. Tested it 3 times on the Test World and the same condition crashes it every time.

1. Setup a storage system (Example picture below)
2. Hopper chain with a chest pointing at the 2nd Inventory Connector.
3. The Hoppers and Chests on the left are Excluded from Inventory Connectors.
4. The Level Emitter set to an item you have in your storage.
5. Put items in the chest on the left that's feeds items into your storage system.
6. Break at least 2 hoppers in the chain, put them back again after breaking them.
7. Then break the 2 hoppers again that you just placed (Crash happens here)

Server Crash Log: https://mclo.gs/qNtcLmt
Test World Crash Log: https://mclo.gs/T7y0ycY

This might be fixed in v2.0.11 at least I can't get it to crash now that I tried that version. Please mark this version on Modrinth as compatible with 1.21.1. Modrinth never shows an update as only v2.0.9 is marked for 1.21.1 compatible. I had been on that version the past few months as the launchers never showed an update.

https://modrinth.com/mod/toms-storage/versions?g=1.21.1&l=fabric