Running on 2.1.7, shows this error when loading up old save ported from 2.0.

The mod Belt Visualizer (2.1.0) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event belt-visualizer::on_tick (ID 0)
LuaEntity doesn't contain key neighbours.
stack traceback:
[C]: in function 'index'
__belt-visualizer/control.lua:278: in function <belt-visualizer/control.lua:276>
(...tail calls...)
belt-visualizer/control.lua:309: in function 'walk_belt'
belt-visualizer/control.lua:324: in function 'cache_belt_line'
belt-visualizer/control.lua:352: in function <belt-visualizer/control.lua:328>

Thanks for the report, fixed in 2.1.1
The docs are missing a lot of things right now, and I missed a new property I was supposed to use for underground belts.