After a lot of back and forth between whether it was Sophisticated Storage causing an issue or Tom's I narrowed it down to Tom's storage.
This is a duplicate of this issue: #505 but I will try to provide a lot more information.

I am running 1.21.1 and these mods:
Cloth Config API; 15.0.140+frabric
Fabric API; 0.115.0+1.21.1
Tom's simple storage; 1.21-2.1.0
(ModernFix 5.20.0+mc1.21.1)
The issue is as follows:

If you enable the keep last item in slot in the inventory connector, put a stack of blocks in the chest, and then click the stack in the storage terminal it will freeze the world. In singleplayer this freezes the world (you can still move around and place blocks but entities are stuck in place without animation), doing save & quit to title will keep the world stuck on saving world, and you need to force close the instance. This produces no log in vanilla. If I install ModernFix; 5.20.0+mc1.21.1 it does generate a log file (its probably describing the freezing of the world). Installing additional more elaborate crash log mods doesnt give any more details
https://mclo.gs/Zv2HskC

In multiplayer this crashes the server and causes the world to be sort of force saved but when restarting the server it can cause it to instantly crash again when the chunk with the storage system is loaded. On the server we initially thought it was an incompatibility between Tom's and certain barrels but above testing proved that it even happened in vanilla. The following log files are from a Cobblemon server running a bunch of mod so I am not sure how helpful they are because I do not know the exact circumstances in which the crash happened. Here it does describe a lot more to what happened
https://mclo.gs/B3wN56J

Rerunning the issue with 1.21.4; and the updates of the mods listed at the top, results in the same issue. Again without ModernFix there is no log file generated and the world freezes and gets stuck on Saving World infinitely.
Cloth Config API; 17.0.144+fabric
Fabric API; 0.115.0+1.21.4
Tom's simple storage; 1.21.4-2.2.0
(ModernFix 5.20.1+mc1.21.4)
https://mclo.gs/AC0jnno

If you need any more information feel free to reach out (not sure what else I could provide at this point), I hope this is fixable and that it does not affect too many players.

tested it on our world and it works, thanks for the fix :) I saw you pushed the update for CursedForge, any chance it can be pushed on Modrinth too? (no idea how the process works, sorry if it is rude to ask :( )

It was pushed on Modrinth: https://modrinth.com/mod/toms-storage/version/1.21-2.1.1-fabric