This happened when I was rotating/deleting belts connected to the belt balancer.

Please report this error to the mod author.
Error while running event belt-balancer::on_nth_tick(8)
belt-balancer/objects/balancer.lua:234: attempt to index local 'lane' (a nil value)
stack traceback:
belt-balancer/objects/balancer.lua:234: in function 'put_on_belts'
belt-balancer/objects/balancer.lua:246: in function 'run'
belt-balancer/helper/message-handler.lua:33: in function <belt-balancer/helper/message-handler.lua:31>
110.662 Error ServerMultiplayerManager.cpp:92: MultiplayerManager failed: "The mod Belt Balancer caused a non-recoverable error.

Where it input or output belts?
Did you removed/added balancer-parts before (especially, did you merged or splitted this balancer before)?
Can you provide a way to reproduce this and your save?
That will help me find and fix this error. If you dont want to post your save here publicy, you can go on github and write me there: https://github.com/knoxfighter