When mining the Underground Pipe Connector. Mining other works fine, it's just this one that's broken.

The mod Pipelayer (0.4.1) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event pipelayer::on_player_mined_entity (ID 65)
pipelayer/Network.lua:108: attempt to index local 'entity' (a nil value)
stack traceback:
pipelayer/Network.lua:108: in function 'for_entity'
pipelayer/Editor.lua:114: in function 'disconnect_underground_pipe'
pipelayer/Editor.lua:391: in function 'on_mined_surface_connector'
pipelayer/Editor.lua:420: in function 'on_player_mined_entity'
pipelayer/control.lua:40: in function <pipelayer/control.lua:40>

Also getting this crash when rotating the entity in Underground view. Rotating surface connector works okay.

The mod Pipelayer (0.4.1) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event pipelayer::on_tick (ID 0)
pipelayer/Network.lua:26: attempt to index local 'entity' (a nil value)
stack traceback:
pipelayer/Network.lua:26: in function 'fill_pipe'
pipelayer/Network.lua:312: in function 'callback'
pipelayer/Network.lua:282: in function 'foreach_connector'
pipelayer/Network.lua:310: in function 'set_fluid'
pipelayer/Network.lua:349: in function 'infer_fluid'
pipelayer/Network.lua:369: in function 'try_to_transfer'
pipelayer/Network.lua:394: in function 'update'
pipelayer/Network.lua:97: in function 'task'
pipelayer/lualib/Scheduler.lua:11: in function 'on_tick'
pipelayer/Network.lua:477: in function 'update_all'
pipelayer/Editor.lua:466: in function 'on_tick'
pipelayer/control.lua:50: in function <pipelayer/control.lua:50>

Can you post the save? It sounds like there may be another mod that has corrupted this particular pipe connector.

Been playing that map since 17 Stable for 350 hours with a growing list of mod so I doubt it will be useful as the error could have been introduced anytime in 2019. However I checked the mods and this one strikes me by far as the most dangerous one https://mods.factorio.com/mod/PickerDollies

Yes, that sounds like a likely suspect. If you can upload the save anyway so I can see exactly in what way it's broken, I can add a migration to the next version to either fix that connector or remove it from the map without crashing.

That would be great, connector in question is two tiles down from player position https://1drv.ms/u/s!Atg8TMIm6TjHoVkpyp3uZpAO5uWX?e=4xhaJW