This bug concerns an incompatibility with Dual Wielding mod. If I recall correctly, this appears to be a relatively new issue. While using these two mods in conjunction, players will be booted from servers, but servers will not crash. This also occurs in Single Player worlds with the "built in" server.

The issue lies with a conflict in the following method, lines 23-26:

https://github.com/SalvieMundi/ImmersivePortalsMod/blob/1.17/imm_ptl_core/src/main/java/qouteall/imm_ptl/core/network/IPCommonNetwork.java#L22

Simply removing those lines in Immersive Portals core makes Dual Wielding mod work with no noticeable problems, although I am sure you have written that code for a reason and there is a better fix. It's also worth noting that Dual Wielding doesn't appear to create entities in a non-server thread, although it does send entity data from client to server and concerns line 33 in this section in specific:

https://github.com/Globox1997/DualWielding/blob/1.17/src/main/java/net/dualwielding/network/PlayerAttackPacket.java

For further info see issue 9 from Dual Wielding here: Globox1997/DualWielding#9

I am going to close this, as it was not IP's fault per the message in the mod, and I have resolved the issue successfully over on the Dual Wielding project. Thanks @qouteall !