The mod Belt Balancer (2.1.3) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event belt-balancer::on_tick (ID 0)
invalid key to 'next'
stack traceback:
[C]: in function 'next'
belt-balancer/control.lua:220: in function <belt-balancer/control.lua:219>

I was using the upgrade planner tool

Error while running event belt-balancer::on_tick (ID 0)
invalid key to 'next'
stack traceback:
[C]: in function 'next'
belt-balancer/control.lua:220: in function <belt-balancer/control.lua:219>

Probably the same thing, I'm using Belt Upgrader. So something about updates around the belt balancer?

Upgrade Planner is the ingame tool, right?
And Belt Upgrader is a mod?
The answer is yes, and i think, i know what is going on. Will look into it tomorrow.

ok, I've looked into it today and i am unable to reproduce this issue. It seems like there are some errors that happens previously and this error just occures later on. So, when your able, please provide a save or a way to reproduce this issue, so i can fix it.

I think I can confirm that it has to do with the upgrade planner. I had this same error happening consistently and found this thread. I removed the upgrade on the belts around the balancers and it stopped happening. I used the planner to upgrade the belts, but it was a small scale upgrade rather than my whole bus slowly over time. It might have to do with how long the upgrade planner is waiting to finish the belts.