Whenever a bot places an underground belt I get the follwing error:

The mod Belt Overflow (1.1.0) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event belt-overflow::on_tick (ID 0)
Entity is not transport-belt-connectable.
stack traceback:
[C]: in function 'get_transport_line'
belt-overflow/control.lua:235: in function <belt-overflow/control.lua:207>

It might be a while before I can look into this.

I think this is fixed now. The problem was with paired ghost-and-not-ghost undergrounds, which could manifest when replacing a ghost with a real underground, or when blueprinting a ghost that connects to an existing real underground.