Not sure if this is specific to Flux Networks or if it is more a bug of SonarCore's packet system: https://pastebin.com/nJzvkkwR the server does not crash from this but the player does get disconnected.
This error occurred seemingly at random so unfortunately I cannot give any steps to reproduce. If I find anything that occurs just before this happens I will update this issue.

SonarCore: 1.12.x-5.0.5
Flux Networks: 1.12.x-3.0.3

Looks like client side is sending packets which are only meant to be sent from server side

Fixed this one in an earlier version