The mod Belt Balancer 2 (2.0.8) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event belt-balancer-2::on_player_mined_entity (ID 68)
belt-balancer-2/objects/part.lua:265: attempt to index local 'belt' (a nil value)
stack traceback:
belt-balancer-2/objects/part.lua:265: in function 'remove'
belt-balancer-2/control.lua:145: in function <belt-balancer-2/control.lua:134>

It happens with only one the Belt Balancers. It crashes when I try to remove it and when I try to upgrade the Underground belt I have feeding it. I was able to remove the Underground I had feeding It but now when I try and put a new Underground or even a normal belt it also crashes.

Having the same issue

Similar issue (but not the exact same error) here:

The mod Belt Balancer 2 (2.0.8) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event belt-balancer-2::on_robot_built_entity (ID 14)
belt-balancer-2/objects/balancer.lua:140: attempt to index local 'stack_belt' (a nil value)
stack traceback:
belt-balancer-2/objects/balancer.lua:140: in function 'recalculate_nth_tick'
belt-balancer-2/objects/belt.lua:218: in function 'built_belt'
belt-balancer-2/control.lua:115: in function <belt-balancer-2/control.lua:94>

It seems the issue is specifically when an underground belt next to a balancer is placed/updated

Fixed by 2.0.9