First Crash Report:

The mod Belt Balancer (2.1.2) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event belt-balancer::on_nth_tick(8)
LuaTransportLine API call when LuaTransportLine was invalid.
stack traceback:
belt-balancer/objects/balancer.lua:198: in function 'run'
belt-balancer/helper/message-handler.lua:33: in function <belt-balancer/helper/message-handler.lua:31>
stack traceback:
[C]: in function 'len'
__belt-balancer/objects/balancer.lua:198: in function 'run'
belt-balancer/helper/message-handler.lua:33: in function <belt-balancer/helper/message-handler.lua:31>

Second Crash Report:

The mod Construction Drones (0.4.11) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event Construction_Drones::on_ai_command_completed (ID 113)
The mod Belt Balancer (2.1.2) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event belt-balancer::script_raised_built (ID 77)
LuaEntity API call when LuaEntity was invalid.
stack traceback:
belt-balancer/objects/balancer.lua:140: in function 'recalculate_nth_tick'
belt-balancer/objects/belt.lua:225: in function 'built_belt'
belt-balancer/control.lua:102: in function <belt-balancer/control.lua:87>
stack traceback:
[C]: in function 'index'
__belt-balancer/objects/balancer.lua:140: in function 'recalculate_nth_tick'
belt-balancer/objects/belt.lua:225: in function 'built_belt'
belt-balancer/control.lua:102: in function <belt-balancer/control.lua:87>
stack traceback:
Construction_Drones/script/construction_drone.lua:1915: in function <Construction_Drones/script/construction_drone.lua:1887>
(...tail calls...)
core/lualib/event_handler.lua:44: in function <core/lualib/event_handler.lua:42>
stack traceback:
[C]: in function 'create_entity'
Construction_Drones/script/construction_drone.lua:1915: in function <Construction_Drones/script/construction_drone.lua:1887>
(...tail calls...)
core/lualib/event_handler.lua:44: in function <core/lualib/event_handler.lua:42>

I can replicate the bug when using the Construction Drones (Mod) and an upgrade planner to change yellow belts to red belts on belts touching belt balancers

Fixed in 1.2.3/2.1.3
Error was, that fast-replacement by scripts, where not handled.
In factorio 0.17, the Contruction Drones mod does not call other mods, when they change entities, so i this still does not work in factorio 0.17 :(